He Who Fights Monsters

""He who fights monsters should see to it that he himself does not become a monster. And if you gaze for long into an abyss, the abyss gazes also into you.""

- Friedrich Nietzsche, Beyond Good and Evil

Sometimes the villain chides the hero by saying the two of them are Not So Different. Usually the hero rejects this claim, but occasionally, it's a valid point, as some heroes degenerate into something that really is Not So Different.

Not exactly a villain, but they act antagonistically enough that they might as well be. Something has happened to our Fallen Hero: his village was destroyed, his friends killed, his puppy roasted on an open spit, his bike stolen, whatever. All that matters is that It's Personal, and he feels that the law just isn't suitable enough (or has become too corrupt) to be of any use to him in settling the matter. Oh, sure, he'll justify his actions by claiming that it's justice he's after, not vengeance, but anyone with half a brain can easily see that he's out for Revenge...unfortunately, we can also see that the more he hunts the cause of his woes, the more he takes on the villain's personality and mannerisms—something that our "hero" is too blinded by his single-minded goal to realize.

He may have good intentions—the fiend may well be too dangerous to be kept alive—but ultimately, his obsession with meting out due punishment twists him into a monster just as bad as, or even worse than, the one he's hunting. And even before he gets to that point, it's nigh-impossible to turn him away; The Power of Friendship and The Power of Love were lost to him the moment the atrocity that sent him on his wild goose chase happened, and he feels that Team Spirit is just a hindrance. Don't expect him to make a Heroic Sacrifice or Heel Face Turn anytime soon; if he dies in the process of bringing his nemesis down, it's usually with him crossing into Villainstown in his moment of glory. If he doesn't die...

The "fighting monsters" line sometimes forms the Moral Event Horizon for heroes, and both antiheroes and Well Intentioned Extremists live close to it, especially the more pitiless Knight Templar and Unfettered types. The Irony that this situation can bring makes this trope useful in writing a Tragedy.

Not to be confused with Complete Monster, although somebody can become this by fighting complete monsters. See also Cycle of Revenge, Protagonist Journey to Villain, You Are What You Hate, and Then Let Me Be Evil. If this trope happens to a child, it can be used as a Freudian Excuse. Compare And Then John Was a Zombie, where the character becomes a literal monster. If You Kill Him You Will Be Just Like Him is pretty much a sped-up version of this. Common in politics, as Reign of Terror, Full-Circle Revolution and Meet the New Boss indicate. If the monster in question is an animal, that's Animal Nemesis. Subtrope of Slowly Slipping Into Evil.

Not to be confused with Those Who Hunt Elves, nor Monster Hunter.

Anime and Manga

 * In Mahou Sensei Negima, Haruna identifies as being one of these. He was an undeniable good guy as a kid, only to grow up into a completely manipulative jerkass little better than his enemy.
 * In a flashback, Tsuruko uses the page quote as part of a lecture to a young Setsuna when showing her the Youto Hina.
 * Ken, the pilot of Blade Gainer in Godannar, is one of these. His obsession with killing the Mimetic Beast that killed his wife causes him to mercilessly hunt every Mimetic Beast down, without regard to how much property damage or civilian casualties he causes.
 * Scar of Fullmetal Alchemist.  Especially ironic since he uses a form of alchemy to destroy his targets: state alchemists.
 * A fact that he lampshaded himself when  Essentially, his thought process was "Well, I'm already cursed... may as well put it to good use."
 * Comes up again when Colonel Mustang . It takes Ed, Scar, and Hawkeye to
 * Hellsing treats this trope in a very interesting way. Nothing is left to interpretation when Alucard shows clear disappointment and grief over
 * Afro Samurai. Reread the first paragraph. That describes, in a nutshell, the entire first season of Afro Samurai.
 * In the Meakashi chapter of Higurashi no Naku Koro ni, and starts on a chain of murders of her own to avenge him, culminating in
 * The monster hunters of Claymore use demonic power to increase their fighting strength. If they use too much, they transform and become living examples of this trope. Special note: here, it's more like "She who fights monsters".
 * Usually it's by accident though, when they over-exceed their limit. However, there was one particular time when
 * Code Geass's Lelouch is fully aware of and, in fact, embraces this trope.
 * Naruto is centered a lot around revenge, mainly on Sasuke's side. He wants to fight his  evil brother and therefore joins the evil side.
 * Arguably, Pain. His ultimate goal was, and if this meant the deaths of thousands, endless war, blood, and conflict, so be it. Also, his monologue about
 * Strangely, Konoha itself is very similar to Pein. From how they treated Naruto in the beginning, it's pretty clear that they aren't the paragons of virtue. As a military government in a feudalistic world, they do pretty shady things to survive and keep peace, including  Danzo, most of all, claims to want Peace by any means necessary, but his main goal is promoting Konoha's power across the world, with him on top, naturally. Danzo is also personally responsible for creating many of Konoha's greatest enemies, by ruining people's lives to the point of wanting revenge on Konoha in the name of the village's safety.
 * Hanzo of the Rain village is also implied to have fallen to this trope; in his youth, he was a Worthy Opponent of Mifune's who spared his life for fighting with honor and dreamed of uniting the ninja world; years later, he's a paranoid dictator whose endless warmongering has turned his country into a god-forsaken hellhole, and who crosses the Moral Event Horizon when he attacks Nagato and Yahiko's peaceful protest group out of fear that they'd try to usurp him.
 * In Yu Yu Hakusho,
 * Also, the Start of Darkness for Yu Yu Hakusho villain Sensui.
 * In Last Exile, Alex Rowe skates dangerously close to this. Although he never makes the jump into full-out villain territory, his need for revenge again Maestro Delphine and the Guild is all-consuming, and at one point, he
 * Now and Then, Here and There: Poor Elamba. He was so devoted to killing King Hamdo he became as dimented and twisted as he was.
 * In Berserk, Guts is every bit as cruel and sadistic as the Apostles he hunts at the beginning of the series. He deliberately tortures the Baron and Count in the beginning chapters, and, after fighting with Rosine, doesn't even stop trying to kill her even after an innocent girl who was tagging along threw herself in front of him. Currently, the only thing preventing him from completely going off the deep end is his need to protect Casca from the monsters he fights, and, occasionally, from himself.
 * And the worst part of this all? If he wasn't willing to sink to that level of depravity, he would have gotten killed a dozen times over. As bad as he is, everything else is worse.
 * As if that weren't enough, due to his unstable personality, which developed in the face of unreasonable adversity, coupled with the effects of this trope, Guts now struggles with a rather powerful inner demon.
 * Ray Lundgren from Gun X Sword. At least the protagonist Van, while skirting this line, still made standards here and there about not ruining properties, taking innocents's lives, etc. Ray, on the other hand, will gladly kill any innocents if they stand between his way and The Claw. He even claims that, just as long as he can kill The Claw, he doesn't mind becoming a sadistic murderer himself.
 * Howl, from Howl's Moving Castle (the movie, not the book), is . Considering her methods, Suliman might qualify, too.
 * Used in One Piece, where one of the Big Bads, Arlong and his crew of fishmen, oppress humans with the Social Darwinist explanation that since they are physically more powerful than humans, they deserve to rule over them. Flip ahead several hundred chapters and it's revealed that they behave this way because they were once horrendously oppressed and enslaved themselves by humans that considered them subhuman and treated them accordingly.
 * Arguably, most of the marines and the World Government too. They claim that they want to dole out justice and protect the innocent, but some of their methods seem even worse than what pirates do in the series.
 * Akainu in particular needs to be mentioned. His very first appearance is in a 20 year flashback where he destroys a massive ship of innocent refugees because there might, just might have been one of his targets on board. For God's sake, even Blackbeard has more empathy than this man.
 * This trope is given a bit of a workout in Monster:
 * In Shakugan no Shana, all flame Hazes aside from the titular character have some elements of this, and it is stated that most of them became flames because they have a personal hatred of the Crimson Denizens.
 * It's the clearest with Margery Daw when she is first introduced trying to kill a Denizen that she knows is not only, but is also under Shana's protection, thus resulting in fighting someone who should be her ally.
 * Char Aznable of Mobile Suit Gundam fits this nicely, up until he acknowledges to his sister that revenge wasn't the best idea later on in the series. However, he turns back to being this in Chars Counterattack.
 * In Mobile Suit Gundam 00,  becomes this as
 * In fact, Celestial Being as a whole qualifies for this trope in the first season.
 * The same thing happens in Gundam AGE, heartbreakingly.
 * Death Note:
 * Light Yagami begins using the supernatural notebook to rid society of criminals, but soon his black list expands to include anyone who stands in his way for any reason, starting with the FBI, and he even declares that he'll start executing people just for being lazy. Along the way, he coolly manipulates the feelings of both people and shinigami. Repeatedly stating that he plans to become the god of the new world he is trying to create doesn't help matters, either. Declaring that he will eventually execute people for being lazy implies that Light has done away with the slippery slope completely and simply jumped off the Moral Event Horizon. Ryuk even lampshades this, but he's just in for the spectacle.
 * Teru Mikami uses the notebook to eliminate minor and reformed criminals.
 * The titular character of Chirin no Suzu, motivated by the senseless killing of his mother by the Wolf King, goes on to become just as ruthless and bloodthirsty as the wolf.
 * Bleach gives us Kaname Tousen, who joins the Big Bad because he wants to seek "justice.".
 * In The Tower of Druaga, whoever reaches the top of the tower, defeats the monster Druaga, and gains the Blue Crystal Rod, by the very nature of the tower, is in danger of becoming Druaga's next incarnation. Fortunately, how fast this happens also depends on the person's nature. Fallen Hero Gilgamesh held out for a century before making his Face Heel Turn, and then, he retained his humanity.
 * The El Baile De La Muerte arc of Black Lagoon is an exploration of this trope in regards to Roberta, who sets out on a Roaring Rampage of Revenge against the people responsible for the kaboom that killed her master, a rampage that has her going Ax Crazy during the course of it..
 * In Yu-Gi-Oh! GX, Judai takes revenge on the Dark World monsters for Johan's supposed death, and ultimately embraces the concept of being evil in order to fight evil, and then proceeds to burn entire villages to the ground, while part of him sits trapped in his own mind, muttering It's All My Fault.
 * Even after his semi-Heel Face Turn, Accelerator in A Certain Magical Index will mercilessly kill anyone who harms innocents, or even worse, try to harm . Granted, he calls himself a villain despite saving innocents, since he knows his rather violent methods are not "heroic", especially compared to Touma, the one Accelerator who fits "the hero" type.
 * In his first appearance in Kirby Right Back At Ya, Knuckle Joe was consumed by hatred in his quest to get revenge on the Star Warrior who killed his dad. This led him to do terrible things, such as trying to kill poor Kirby who wouldn't even fight back. Then cue the appearance of Meta Knight, who reveals that he was the one who killed his father and calls him a Demon Beast/monster for the awful acts he committed, as well as his abandoning of reason. This was the perfect opportunity for Nightmare, who later turned Joe into a monster that could fire spikes at the cute pink spud. See? Not all monsters are made by Nightmare himself - it's also possible to become one by abandoning reason rather than listen to it as well as living only by hatred.
 * In Holyland, Yuu struggles with the fear of going too far as a Bully Hunter and becoming no better than the thugs he battles. Various characters attempt to reassure him otherwise.
 * Children Who Chase Lost Voices: The topsiders may have started the conflict with their plunder of Agartha, but by the time the film occurs, the latter are no more some saintly oppressed folk, but have become violently xenophobic. They exile their own for contact with topsiders, at least one has stones thrown at her by Agarthan children while the adults just stand by and let it happen, and a cursed tribe of mutants exist that is tasked with hunting and murdering
 * Children Who Chase Lost Voices: The topsiders may have started the conflict with their plunder of Agartha, but by the time the film occurs, the latter are no more some saintly oppressed folk, but have become violently xenophobic. They exile their own for contact with topsiders, at least one has stones thrown at her by Agarthan children while the adults just stand by and let it happen, and a cursed tribe of mutants exist that is tasked with hunting and murdering

Comic Books

 * Batman's greatest fear is that he will become this, if he hasn't already. In fact, this is the way many other heroes see him, and they are not entirely wrong (depending on who's writing him).
 * It's also why Batman so strictly adheres to Thou Shalt Not Kill: having that as a line that he never crosses is a barrier to slipping over the edge and becoming as much of a monster as the psychos he fights.
 * Mr. Freeze perfectly represents this fear and this trope in the animated series with the gusto he goes after those responsible for what happened to his wife, Nora. Freeze wants vengeance at any and all costs.
 * The Punisher of Marvel Comics is often presented this way whenever he makes a guest appearance in more idealistic books like Spider-Man or Daredevil. However, in his own book, it's a very rare problem that isn't best solved by shooting it in the face.
 * Of course folks like Spider-Man and Daredevil have expressed this opinion of him more than once, and have often tried to take him down. However, Frank told Spider-Man to his face once that Spidey could have wound up the same and still could, especially if he didn't have super-powers; Spidey didn't get the point, as clear as it was.
 * Even in his own books, he's portrayed as a profoundly messed up individual, more tortured machine than man.
 * The Vigilante, Adrian Chase, slew himself for this very reason.
 * The Lone Wolf, in Mike Barry's novels, ended up going so out of control that his own sidekick took him out. Mike Barry, actually Barry Malzberg, felt pleased to bring the series to this conclusion.
 * And yet, he still worries. After 'The Slaver' storyline, he is troubled by the graphic extremes he went to rescue innocents and dispose of the bad guys. He just finished shooting some people in the forehead...
 * In X-Men comics and especially the movie trilogy, Magneto - a survivor of the Holocaust - is so determined to ensure that what happened to him never happens to his fellow mutants that he becomes increasingly xenophobic and genocidal towards unpowered humans, quite happy to wipe them out in order to ensure mutantkind's supremacy, and ultimately winds up little better than those who prompted him to begin his fight.
 * A fact that former Nazi Red Skull pointed out. It should be worth noting, though, that Red Skull was sincerely trying to praise Magneto. He somehow didn't know that he was a Holocaust survivor.
 * Rorschach from Watchmen describes this in great detail, recounting how he became a dark and gritty Anti-Hero first (though he had a violent childhood) by brutally taking out his anger and disgust on a by  It even affects Rorschach's psychologist. The chapter in which we learn this is even called "The Abyss Gazes Also" and ends with the Nietzsche quote above.
 * Indeed, this trope also features in the Tales of the Black Freighter sub-comic: a lone, marooned sailor, convinced the titular ship will raze his village in his absence, returns to defend his loved ones on a raft of his mates's bloated corpses. He begins his bloody crusade against the raiders --
 * Ozymandias could count toward this as well, seeing as
 * General "Thunderbolt" Ross from Incredible Hulk. This is made especially clear in Hulk: Gray, where many parallels between Ross and the Hulk are drawn and Ross grows more and more fanatical in his pursuit of the Hulk as time goes on. Eventually, in his pursuit to defeat him, he became what he hunted: . He even lampshades it.
 * Spider-Man villain Supercharger was the son of a scientist who was obsessed with mapping the biology of superheroes and was given electric powers in the very accident that killed his father. So, the guy was embittered against superheroes and felt that they were ultimately more trouble than they were worth. Somewhat understandable. So he demonstrates that people with superpowers are dangerous by going completely crazy with his powers so people will see how dangerous he is.
 * The Jedi Covenant from the Knights of the Old Republic comics becomes so determined to stop the Sith from re-emerging that they're willing to kill their own apprentices..
 * Elf Quest has the Go-Backs as an example of a whole culture falling prey to this. All they wanted was to follow the call of the souls of their ancestors to the Palace of the High Ones, but the Frozen Mountain Trolls fought a war to keep them away from it. The Go-Backs didn't only grow into ruthless warriors, but also took up a habit of their enemies that disgusted even other trolls: eating the bodies of the enemies they killed in battle.
 * The Red Lanterns, especially Laira, who was the first Green Lantern to use the newly-written premise to kill for revenge, which was why she was expelled from the Green Lantern Corps. Feeling betrayed, she became a perfect host for a Red Lantern Ring, which then turned her into as dangerous and murderous a psychopath as many of the Sinestro Corps members that she hated so much.
 * Sinestro is this even more. To bring order to his planet, he turned it into a totalitarian regime, becoming the symbol of tyranny in a star system. Once Hal Jordan took him down, he decided that Guardians are not up to the job of bringing order and defeating crime in the Universe and swore to destroy the Green Lantern Corps and replace it with his own order. He has no problems teaming up with several DC Universe villains and, in the end, created his own corps, openly acknowledging that most of it's members are Complete Monsters of the same kind as those he once fought..
 * Guardians tried so hard to ensure GLC victory in the war with the other Corps that they established several oppressive rules, allowing Corps members to kill and banning relationships between the members. They had no problems with giving Larfleeze a star system and giving him the location of the Blue Lanterns's (who wanted to become their allies) headquarters to make him stay away from rest of the Universe.
 * Rick Grimes from The Walking Dead is becoming more like this as the series progresses. His actions have become increasingly brutal as his obsession with keeping his son and the group safe grows. He himself is beginning to recognize this however, although we'll have to wait and see if he can pull himself back from the brink.
 * Megatron in the IDW comic books was this. He initially formed the Decepticons to try and bring equality and justice to Cybertron, the Cybertronian society having become so corrupt and fascistic that Autobot thugs freely handed out beatings to innocent Cybertronians for no reason and the Autobot senate's answer to dealing with peaceful protests was to have all of the protestors shot. However, over the course of four millenia of war, as well as the brutal things he did to gain power in the first place, he's become exactly the kind of heartless and vicious tyrant that he originally despised.
 * While Superman rarely comes close to this trope, he is very aware of it and lampshades it on several occasions, mostly in response to Anti Heroes who are just as bad as the villains they fight. He even occasionally criticizes his own actions in the newspapers as Clark Kent when he does veer dangerously close to this territory as Superman.
 * This fear is what made him give a piece of kryptonite to Batman.
 * Reed Richards of the Fantastic Four blatantly averts this trope when dealing with his Arch Enemy, Doctor Doom. To this day, he still doesn't outright hate Doom, no matter what horrible things Doom inflicts upon him. In fact, in one storyline where he was exceptionally pissed off, Reed told himself that he really had to kill Doom this time, only to break down in grief and pity when he confronted him, and wound up pleading with Doom to reform.
 * However, when one of Doom's schemes backfired and resulted in him being trapped in Hell, Reed tried to turn Latveria into a democratic country and quickly ended up using Doombots to stop anti-government riots.
 * In Kingdom Come, Orion has killed Darkseid and became ruler of Apokolips. In order to maintain peace, he has to make many questionable choices and Superman notes that he is now more like his father than ever before.
 * It only skirts this if you assume that he isn't trying to justify his becoming this and was telling the truth. Orion states that he tried to make Apokolips a better place. The residents couldn't handle freedom. This forced Orion to become more like Darkseid in order to keep the population under control.
 * In Harry Kipling (Deceased), the New Atheist Militia is inspired by Kipling's example and sets about killing gods - by committing genocide against their followers. In fact, their denial is so intense and fanatical that it manifests as what Kipling describes as an 'anti-god', and the NAM is as trapped by their own anti-god as the believers are by their deities.
 * Alien vs. Predator: The Web has a survivor of the mess on Ryushi (back in Aliens vs. Predator: Prey there was a colony, which had to be evacuated under attacks by Yautja and Xenomorphs they seeded as a "game preserve"). Now he's a multi-billionaire and kills the whole Yautja hunting parties in a trap, using as bait fake Distress Calls, partially terraformed planet and Xenomorphs he grows specifically for this purpose. The dude first thought of this as avenging his parents, but now he admits it's just for fun.

Fan Works
""No! I must kill the demons" he shouted. The radio said "No, John. You are the demons"."
 * A very easy trap for a protagonist of a Mary Sue Hunter fic to fall into.
 * In Aeon Natum Engel, a Nazzadi (artificially created human-like race created for the sole purpose of annihilating humanity, but rebelled) is supervising a project of Replica soldiers (artificially created soldiers for sole purpose of annihilating the enemies of humanity, including the Nazzadi's creators, the Migou, and they have no concept of rebellion).
 * Doom: Repercussions of Evil


 * One criticism of The Open Door is that while the Necrons are Omnicidal Maniacs, the nominal Type V Anti Heroes/Villain Protagonists are themselves highly culturally and militarily aggressive. Their aspiration to and failure to achieve The Unfettered status only serves to blacken their image further, in the eyes of some.
 * A central theme in Avatar: The Last Airbender Revised is preventing oneself from succumbing to He Who Fights Monsters by only killing when absolute necessary. This is present primarily in Katara's character arc, although it fits into those of others as well. A paraphrase of Nietzsche's quote is used as the Arc Words for the first three books to reflect this.
 * Not only the theme but the name of He Who Fights Monsters, a Rosario + Vampire story where Tsukune never meets Moka or the others. To keep anyone from finding out that he's human (and thus stay alive), he's eaten bugs, made several Improvised Weapons, and . "When your life's on the line, there's nothing that you won't do to survive."
 * Ponies Make War: After Twilight Sparkle is freed from Nihilus' control, she becomes an Actual Pacifist, as she fears that this trope will come into play—she was forced to watch Nihilus perform countless acts of violence and torture For the Evulz, and is afraid that if she starts fighting those responsible for her transformation, she'll start enjoying it.

Film
"Batman: There is a difference between you and me. We both looked into the abyss, but when it looked back at us... you blinked."
 * Knockout Ned from City of God slowly turns into this after his girlfriend was raped and his house was shot up.
 * Played with in the movie Ravenous. The villain gained power through cannibalism and the only way the protagonist can fight him is by partaking in cannibalism himself.
 * Sweeney Todd: The Demon Barber of Fleet Street, who starts the movie only wanting revenge upon Judge Turpin for sending him away to Australia and raping his wife, starts committing the murders that would mark him as the Demon Barber of Fleet Street after "Epiphany" and "A Little Priest", finally going off the deep end at the climax of the movie when, . We've got a serious Downer Ending here.
 * Justice League: Crisis on Two Earths. Batman references the Nietzsche quote when he gives a Shut UP, Hannibal/"The Reason You Suck" Speech to his counterpart Owlman:

"Alfred: Vengeance blackens the soul, Bruce. I've always feared that you would become that which you fought against. You walk the edge of that abyss every night, but you haven't fallen in and I thank heaven for that."
 * Batman: Mask of the Phantasm: Alfred also refers to the Nietzschian quote.


 * And this is what happened to.

": You either die a hero, or live long enough to see yourself become the villain."
 * Batman: Under the Red Hood, like Phantasm, examines this Trope.
 * in The Dark Knight Saga goes so far as to Foreshadowing this with a line similar to Nietzsche's.


 * Batman Forever has one almost as good when Bruce lectured Dick about what would happen if he killed Two-Face.

""You make the kill. But your pain doesn't die with Harvey, it grows. So you run out into the night to find another face, and another, and another... until one terrible morning, you wake up and realize that revenge has become your whole life. And you won't know why.""


 * Played straight with Michael Keaton's interpretation of Batman in Batman Returns, who doesn't adhere to the "No Kill" policy to the point of smiling when he killed a fat guy via dynamite.

"Yuri: "I guess they can't own up to what they usually are: a federation of worse oppressors than the last bunch of oppressors. Often, the most barbaric atrocities occur when both combatants proclaim themselves freedom-fighters.""
 * In Natural Born Killers, serial-killer-obsessed Jack Scagnetti, who is tracking the film's Villain Protagonist duo, eventually.
 * Van Zan from Reign of Fire will do anything necessary to bring down the male dragon, including press-ganging members of Quinn's homestead when not enough of them volunteer.
 * Kevin Bacon's character from the 2007 film Death Sentence. Lampshaded by the gang leader in the ending, who, in a Not So Different moment, revels at the fact that he has reduced the protagonist to such a pathetic shell of a man.
 * All the King's Men in a nutshell: Willie Stark counters corrupt politicians but becomes one in the process.
 * PG-13 teen comedy example: Cady of Mean Girls starts off infiltrating a Girl Posse to get revenge on the Alpha Bitch, Regina, but over the course of the movie, eventually becomes a cruel, mean-spirited Alpha Bitch herself.
 * Steven Spielberg's Munich controversially depicts "Operation Wrath of God", the covert assassination operation conducted by the Israeli government in retaliation for the murder of 11 Israeli athletes at the 1972 Summer Olympics in Munich, as being an example of this.
 * Thirteen Days. If memory serves, one of Kennedy's military leaders, saying that he's been "at war with the Russkies for twenty years", advises Kennedy to deal with the Cuban Missile Crisis, essentially, by invading Cuba and hoping that Russia doesn't have the nerve to let the retaliation lead up to nuclear war. His justification was that "the only thing [the Russians] understand is force."
 * Probably Curtis LeMay—who was like this in real life, according to Robert McNamara in the documentary, Fog Of War.
 * A good guess (considering LeMay's personality), but in the film, it was Dean Acheson who spoke the line.
 * Will Graham in Manhunter and Red Dragon is a criminal profiler who lives in fear that his understanding of the mind of a killer will turn him into a sociopath.
 * Sheriff John Quincy Wydell in The Devil's Rejects is this trope to a 'T'.
 * The Element of Crime is a particularly dark and cruel example of this trope. Thinking like a criminal is perhaps not such a good profiling method when the criminal is a child killer.
 * This is one of the most important themes in Park Chan-Wook's Vengeance trilogy.
 * In Sympathy for Mr. Vengeance, the father sees his daughter on a coroner's slab and nearly faints. Later, after he's resolved to revenge, he watches the autopsy of another child and yawns.
 * Most obvious in Oldboy. Both Oh Dae-Su and his target Lee Woo-Jin are aware of the trope. Oh plainly states, "Now I have become a monster." Lee utters the meaningful line, "Farewell, Oh Dae-Su," and refers to him as "Mr Monster" from then on.
 * In Lady Vengeance, it is a crucial element of the plot. The movie doesn't end with the main character's revenge, but with her realisation of what she has become, and her vow to finally clear herself of sin.
 * Yuri Orlov in Lord of War remarks upon this in reference to the revolutions in Africa:

"Groeteschele: How long would the Nazis have kept it up, General, if every Jew they came after had met them with a gun in his hand? But I learned from them, General Black. Oh, I learned. Black: You learned too well, Professor. You learned so well that now there's no difference between you and what you want to kill."
 * Sarah Connor in Terminator 2. Her personal mission was to stop Judgment Day from occurring and keep Skynet from forming. To accomplish this, she hid from the law, raised her only child to become a military leader, shacked up with gun runners, and traded sex for stockpiles of illegal weapons, and, at her worst, almost killed an innocent man (whose life's work caused Judgment Day to happen) in front of his wife and son.
 * That last point is particularly important, because she was trying to kill what is essentially the "father" of Skynet before he could create it. Which is exactly what Skynet sent the first terminator to do to her before she could "create" John.
 * In Fail-Safe, Professor Groeteschele is arguing with Air Force General Black over the merits of launching a first-strike nuclear attack against the Soviet Union in the wake of a technical malfunction that sent a U.S. bomber to drop a bomb on Moscow, arguing that the threat posed by Communism justified it.

"Yoda: If you choose the quick and easier path as Vader did, you will become an agent of evil, and the galaxy will plunge into the abyss of hate and despair."
 * The Anti-Villain from Law Abiding Citizen. In fact, it seems like illustrating this point was his entire plan (beyond the immediate revenge on the men who raped and murdered his family). Just look at his satisfied reaction to be turned down for a plea or any sort of deal at the end. He wanted to shake the system so hard, the broken bits would be exposed, showing everyone who would look the kind of monsters that people become and allow when they start making compromises on Justice.
 * Also, he was nutso-bonkers.
 * Red Hill has this as an important part of the plot.
 * Referenced by the Free French member Private Leroux—aptly nicknamed "Frenchie"—of the Ragtag Bunch of Misfits in the 1995 remake of the movie Sahara. He was sent by La Résistance to Africa to fight as a regular soldier since he had "too much hate [for the Germans] to make good Resistance."
 * Inglourious Basterds had the eponymous Basterds treating the Nazis in the same manner that they do the Jewish people.
 * F.I.S.T.: over the course of the film, the titular labor union begins to act more and more like the Corrupt Corporate Executives they were striking against in the beginning. This is driven home in the scene where they break up a wildcat strike (a strike unauthorized by the rest of the union) by force.
 * By the fifth Halloween film, Dr. Loomis is ready to use blackmail, threats, and physical force to make sure Michael Myers is put down for good. It goes so far that he uses Jamie, a 9 year old girl, as bait to lure Michael into a trap, and then beats him savagely with a plank of wood until Michael was unconscious. And yet he continues to beat him, all while screaming "DIE! DIE!" for each hit.
 * Star Wars: The Empire Strikes Back: the Darth Vader with the face of Luke Skywalker inside the dark side infested cave of Dagobah is Yoda's warning that Luke can become a monster just like Vader should he give in to the dark side, no matter the reasons.


 * And in Return of the Jedi, the moment when Luke strikes off Vader's mechanical hand and then stares at his own is when he realizes that he's right at the edge of that abyss, with his toes hanging over.

"Lily: Jean-Luc, blow up the damn ship! Picard: No! NOOOOOOOOOOOOO!!!"
 * Magneto in X-Men: First Class notes that he and Shaw are exactly the same and admits that he would work with him...except that Shaw killed his mother. And that's the only reason he's going to kill him, not to save humanity or stop Shaw's plans.
 * Star Trek: First Contact: Picard is so blinded by rage and vengeance that he becomes as cold and unsympathetic as the enemy he's fighting, to the point of alienating (heh) his officers and refusing to consider the plan that represents their best chance of success.


 * Serenity: The Operative is fully aware that in his quest to kill monsters he's fallen into this, and states that on his list of monsters, he's right there at the bottom of the list and will kill himself when he's done.
 * Tyler Durden of Fight Club. By the end, he's shaped his group to be just as conformist as the consumerist society he's trying to overthrow, and in some cases, it's even worse.

Literature
"It was a strange, chilling thought, dancing across his head like a white thread against the - terrible red background. It went on: He can think like you. You must think like him. But if I think like him, he wins, he thought back. And his new thought replied: Why? To think like him is not to be him! The hunter learns the ways of the hog, but he is not bacon. He learns the way of the weather, but he is not a cloud. And when the venomous beast charges at him, he remembers who is the hunter, and who is the hunted!" "How it came to this? How we who hated missionaries became missionaries ourselves before we knew! Missionaries of rocket launchers..."
 * As mentioned in film section above, Willie Stark from All the King's Men becomes the kind of politician he once meant to oppose.
 * Bartemius Crouch, Sr. from Harry Potter, the head Auror (dark wizard fighter) during the first Wizard War, was later depicted as "having become almost as bad as those he was fighting", authorizing torture and the use of lethal force.
 * Captain Ahab in Moby Dick, making this Older Than Radio.
 * Subverted Trope in A Series of Unfortunate Events, in which the quotation of the above aphorism is enough to convince the heroes not to drop a villain into a literal abyss.
 * Robert Neville, protagonist of Richard Matheson's famous vampire novel I Am Legend, is seen this way for killing both the feral vampires and those who have enough sanity to attempt a rebuilding of society. In his defence, he didn't have a way or an incentive to tell the difference.
 * The original ending of The Movie had him discover that . While in the theatrical ending,, which, in light of the original ending, fits this trope without actually intending to.
 * The novel ended with
 * The Eisenhorn novels set in the Warhammer 40,000 universe chronicle, in first person, the struggle of Inquisitor Eisenhorn against the vile forces of Chaos while attempting to avoid being corrupted by them himself. As the series progresses, he shows himself more and more willing to use the devices of Chaos against itself, applying a sort of "ends justifying the means" logic to his actions.
 * The danger of this happening is given as one of the reasons that the Imperium's Inquisition is so prone to slaughter everyone associated with an outbreak of Chaos, sometimes including the soldiers who helped them fight against it, since association with most enemies of the Imperium, but particularly Chaos, damns one in the eyes of the authorities. The Gaunt's Ghosts novels also deal heavily with this trope, particularly in Traitor General and the later novels.
 * This trope is, in fact, held to be inevitable by Eisenhorn and his protege, Ravenor. Ravenor explicitly stated that the Jump Off The Slippery Slope is inevitable when you spend so long fighting monsters - what matters is how much good you do beforehand.
 * In Do Androids Dream of Electric Sheep?, the book that inspired the film Blade Runner, Decker meets a fellow bounty hunter, Phil Resch, who hates Replicants a good deal more than he does.
 * This trope is a central theme of the book. Deckard often doubts that he is any more human than the replicants he is hunting.
 * Which, appropriately enough, makes him one of the most human characters in the story.
 * The philosophy teaching love interest in Jeffery Deaver's Garden of Beasts mused on the eponymous Nietzsche quotation with reference to the hitman main character.
 * Subverted in Terry Pratchett's non-Discworld novel, Nation. The heroine has been warning the hero about the Big Bad, who, as typical in Pratchett, is far more of a monster than any creature with a face full of tentacles. In the middle of fighting said Big Bad, this goes through the hero's head:
 * Madame Atomos, the most famous creation of writer André Caroff, is a Yellow Peril villain whose goal is to take revenge on America for the many lives lost during the bombings of Hiroshima and Nagasaki. This includes Atomos's husband and children.
 * Missionaries, by Lyubov and Yevgeny Lukin. Nerdy guys found a portal into the past (as they thought) and tried to stop an European colonization via giving the locals-to-be-colonized somewhat more advanced weaponry. They succeed... but the local development was a bit faster and not in the way than they imagined—and the European exploration slower. So it ended up much the same way, only with the roles changed, "ethanol-powered turbine polymaran rocket plane carriers vs. caravels" being an obvious Curb Stomp Battle.


 * In Codex Alera, by Jim Butcher, it turns out that Attis Aquitane, one of the villains trying to overthrow Gaius Sextus the First Lord, came to be where he is because
 * Mordeth from The Wheel of Time series.
 * The Children of the Light also fit: originally, they were created to find and destroy Darkfriends, but by the time the story takes place, they have begun to persecute anyone who can use the Power. It Gets Worse.
 * And Rand himself almost invokes this trope, until.
 * In the Star Wars Expanded Universe, Jacen Solo eventually comes to the conclusion that the only way to save the galaxy from an evil dictator is to attempt to become one himself. This is one of the leading reasons for his fall to the dark side and conversion to Sith.
 * This is fairly common throughout the Star Wars EU - namely, characters concluding that the power of the dark side is the answer in order to defeat a greater evil. See
 * A quote attributed to Yoda is a paraphrase of the page quote: "When you look at the dark side, careful you must be… For the dark side looks back."
 * This is also what led Anakin Skywalker to become Darth Vader.
 * At the end of the Warhammer 40,000 Grey Knights novel Hammer of Daemons, Alaric expresses concern that the plan he concocted to bring down Drakaasi's Chaos lords and escape makes him less of a Grey Knight. He even outright compares himself to a rebellion-fomenting cultist.
 * Fëanor from The Silmarillion.
 * The Kingpriest from Dragonlance started out as The Messiah, but as he became increasingly confronted with corruption in the world, his quest to purify it became more and more unhinged. In the end, he was hardly better than the people he was fighting and was completely insane to boot. The scary thing was, up to the very end, he was still charismatic enough to convince people that he was still the same kind and pious man who took the throne decades ago.
 * This is the fate of  in the Liveship Trader trilogy by Robin Hobb. He's an interesting case in that most of the monster fighting took place before the trilogy starts, and is only revealed in flashbacks. So, severe overlap with Freudian Excuse in this case.
 * Robert S. Pierre and Oscar Saint-Just in the Honor Harrington series, although it is very difficult to realize this. Unlike most examples, these two were never "good guys", but they were originally explicit in their devotion to Pragmatic Villainy Villainy. They did not oppose the Legislaturalists because they were an evil regime, but because they were an incompetent regime that could not adequately govern the state of the Peoples Republic of Haven, which was on a fast track to complete collapse. Saint-Just himself says that he does not care who holds power, or what they use it for, as long as they use it well. However, once they staged their coup and assumed complete control, they began to fall into the exact same traps and patterns of the Legislaturalists, including the promotion of personnel based on their political connections instead of skill, and the forgiveness of their errors because of those same connections. That specific action was one of the final straws that instigated their coup, but their planned reforms and actions are slowly pushed further and further into the background as they become more and more preoccupied with simply maintaining control.
 * Saint-Just ends up well into Complete Monster territory for his actions as head of State Sec and for . But with Pierre, we get enough of a portrait of his origins, original idealistic intentions, and failure to fix a system that just can't be fixed, that we feel somewhat bad when he dies. The Pritchart administration even admits that Pierre's policies as Chairman improved the Republic's economy and education in the long run, which redeems him a bit further.
 * Unsurprising, as the series is glossing the French Revolution, which has its own entry under "Real Life".
 * In Philip Pullman's His Dark Materials, two of the major nemeses—Lord Asriel and Marisa Coulter—embody Sydney Harris's maxim (quoted above) over the course of the trilogy.
 * It's implied (or at least believed by non-witchers) that the three main causes of death for Witchers are monsters, angry/scared peasants (as a monster), and other Witchers.
 * In Jim Butcher's Dresden Files, the book Fool Moon has.
 * The main character arc of Anita Blake over the course of her eponymous series was her either becoming a sociopathic serial killer who just happened to have a socially acceptable victim profile or realising that she was this all along. At the end of the last book of the series, Otto, another serial killer, acknowledges her as a colleague, and she can't really deny it.
 * Crixus in Emperor: The Death of Kings comes close - he intends to take his revenge on the wealthy Romans by taking the slave army to live in their grand houses. Spartacus points out that since those big houses and farms need slaves to maintain them, living in one would make Crixus just as bad as the senators he hates so much.
 * In the Hunger Games trilogy, does a bit of this with their rules and war tactics. Katniss also calls  out on his tactics and standards that are very similar to that of their enemies. Also happens to   when they decide to
 * To be fair,  was pretty horrified by that idea.
 * In The Saga of Darren Shan, this is how Evanna predicts  Start of Darkness as the Lord of the Shadows,
 * A major theme of Animorphs. The protagonists often worry about how to defeat their enemies without becoming like them
 * Alloran, who committed genocide to prevent the Hork Bajir from falling into Yeerk hands, is said to have been an idealist before the Yeerks's betrayal and slaughter of his comrades drove him over the Despair Event Horizon.
 * Twenty Thousand Leagues Under the Sea: Captain Nemo Majored in Western Hypocrisy and wanted revenge against The Empire. He creates an NGO Superpower with an Oddly Small Organization with its own Con Lang. He claims a continent in his name, creates the Nautilus to conquer The Final Frontier (the sea) and to use it as a Weapon of Mass Destruction, insists on only using sea-related products, and the prisoners he considers valuable are placed in a Gilded Cage, but those who are not are mercilessly destroyed. In trying to destroy The Empire, he ends up creating a society very much like it.
 * The Doctor Who New Adventures novels, which featured a Doctor who, in the name of defending the universe from evil, would not only Shoot the Dog but subvert history over a hundred years to make sure the dog and the gun were in the right place, and then blow up the planet just to be on the safe side, often contemplated the Nietzsche quote.
 * Hercule Poirot knew he possessed both the ability and the ego to become the very kind of serial killer he was always working to put behind bars.
 * Sisterhood series by Fern Michaels: The Vigilantes are a group of women and one man who have been wronged in a number of ways. They decided to get Revenge on those who wronged them. That's right, they're not really pretending to be doing this for justice! They definitely have this happen to them in Vendetta by skinning John Chai alive. He was a creep and a Smug Snake, but the Disproportionate Retribution inflicted on him caused the group to sink to his level! To make things even creepier, the author consistently portraying Revenge as a good thing with no bad effects, the author consistently trying to make the protagonists seem good and heroic despite their actions saying otherwise really sabotage the series!

Live Action TV
""Which side are we on? We're on the side of the demons, Chief. We're evil men in the gardens of paradise, sent by the forces of death to spread devastation and destruction wherever we go. I'm surprised you didn't know that.""
 * One episode of The X-Files featured an FBI agent who became so obsessed with "getting into the head" of a serial killer that he was chasing that he soon turned evil and began murdering people in a manner similar to that used by the serial killer.
 * [Broken Pedestal|This was Mulder's former boss, and Mulder wound up having to stop him]].
 * Dark Willow from Buffy the Vampire Slayer started out wanting revenge against Warren and his friends for what they did to her girlfriend, Tara, but her rampage soon escalated into Card-Carrying Villain territory, until she eventually tried to destroy the world. But unlike most villains who take this road, Willow was brought back from the abyss by Xander.
 * Warren's friends didn't have anything to do with Tara's murder - it was all Warren's doing. Buffy pointed out that while Warren might have deserved to die, Jonathan and Andrew didn't - if Willow had killed them, she would definitely have been unable to forgive herself later.
 * Debatable. Jonathan and Andrew were accessories to Warren killing Katrina, helped him frame Buffy for said murder, were accomplices that drugged Buffy which led to the Scoobies' near deaths by an out of control Slayer, and, in general, made life living hell for the Scoobies for months and months, and this team endangered multiple people in Sunnydale with their actions. They played a part in Warren going out of control and could have prevented Warren from killing two people and nearly killing another along with his various other actions.
 * While Jonathan and Andrew unquestionably share liability for many of Warren's earlier crimes, as they were accomplices, they have no liability for Tara's death. Warren made the choice to go shooting at Buffy's house of his own free will, and he did it entirely by himself. Indeed, Jonathan and Andrew were already in jail at that point.
 * Also, the whole "destroy the world thing" was motivated by an influx of positive magical energy that she drained out of Giles (who had borrowed it). This upped her powers to the point where she could feel an empathic connection to almost every living thing on the planet, but all the dark energy inside her and her own pain and grief left her overwhelmed by all the suffering and pain. She decided that destroying the world would be an act of mercy for everyone involved.
 * Holtz from Angel. He is so obsessed with obtaining "justice" against Angelus that he followed him into the future, disregarded all the myriad evidence of Angel's reformation, and did all he can to make Angel suffer psychologically, to the point of
 * Angel himself goes pretty far into this territory in season 2, and he seems to do it deliberately, re-shaping himself into someone willing to use evil methods to wipe out evil.
 * Very many Cybermen and Daleks the Technical Pacifist Doctor has killed on Doctor Who. He has annihilated entire fleets of enemy spacecrafts and, presumably, his own people. Don't leave out the various monsters of the week. The Doctor seems to swing back and forth on the Sliding Scale of Idealism Versus Cynicism quite frequently. In one case, the Doctor was attacked by creatures who wanted to steal his immortality. They got their immortality all right. Getting the Doctor personally angry is, in his own words, "not a good place to stand."
 * As Donna says in "The Runaway Bride", "I think sometimes you need somebody to stop you." - since he became the Last of His Kind, the influence of a companion ideally should serve to keep him from becoming too monstrous.
 * In the Tenth Doctor's final TV appearance, "The End of Time", they go into detail about what was going on at the end of the time war.  If the Doctor hadn't ended it the way he did,.
 * The Eleventh Doctor has been told that other people are viewing him like this. See the episodes "The Pandorica Opens" and "A Good Man Goes To War" for specific examples.
 * Everyone in Supernatural has this problem all the time. It's not just the contact-with-evil, that is, the 'Monsters' part; it's also the 'Hunts' part, the violence inherent in the lifestyle.
 * Gordon Walker is the purest example, becoming worse than the monsters he hunts taking them out. For a series that can succumb to the temptation of explicitly spelling out character psychology as frequently as Supernatural (how many times has someone told Dean that he lacks self-esteem, is afraid of being alone, is dead inside, yadda yadda yadda), Gordon was thankfully handled with restraint. In his three individual episodes, he comes off as just a sadistic bastard, but put them together and the story is all there:, refusing to admit that it was out of anger instead of necessity. But inside, he is so guilt-ridden that he is desperate for everyone to see the world in terms of black-and-white (which would justify his actions), with Gordon on the side of the good guys (thus his creepy obsession with getting Dean's approval).
 * All the Winchesters have been like this (mixed in with that good old Death Seeker attitude) at some point. John was this way about everything related to Mary's death.
 * Dean was like this this after John died and he had that big-secret-that-totally-wasn't weighing on his shoulders, and has had such moments of ruthlessness every time his family leaves him or lets him down or he's really freaking out about his brother. Such as when he encounters Gordon in season two after his father dies; when he so loses faith in his brother that he agrees to the angels's plan in season five even though it will destroy most of the world; and in season seven when he
 * Sam was this after Dean died in Mystery Spot and the season three finale. While he thinks killing Lilith is the only way to prevent the Apocalypse and feeding demon-blood-fueled powers also lets him save the hosts when exorcising demons, his obsession with gaining the power to kill Lilith.
 * Future Dean in "The End" (5x04). After losing his brother and failing to stop the apocalypse, he becomes heartless and unsympathetic, willing to sacrifice all of his loyal friends for a chance to kill Lucifer.
 * In seasons 6 and 7, has taken a particularly nasty route to this, starting with a Deal with the Devil, moving on to murder and betrayal, and then Jumping Off the Slippery Slope with murder and Mind Rape of friends even before diving into With Great Power Comes Great Insanity.
 * Eric van Helsing from Young Dracula is a comedic version of this.
 * Dexter's eponymous Villain Protagonist inverts this as his fight makes him less of a monster than he would be otherwise. He's more a combination of Evil Versus Evil and Even Evil Has Standards.
 * , however, falls into this after his brother is killed, leading him to find out Dexter's secret and learn his methods, leading him to  and
 * HRG in Heroes. While much of what he does is for Claire, working to capture the monsters in Level 5 shaped him into the unscrupulous operative he is today
 * Peter is also headed down this path in Season 3 of Heroes, when.
 * One season 9 episode of Law and Order Special Victims Unit had a serial killer found dead in the same manner as his victims.
 * Seems pretty standard for Law and Order characters. Sometimes, the rules screw them, though. Like Law and Order SVU's Chester Lake.
 * This was the origin of the title character in Xena: Warrior Princess. Xena first raised an army to protect her village from a warlord, but her brother was killed in the process. She proceeded to actively seek out possible enemies of Amphipolis and destroy them; it was not until her first encounter with Caesar that she abandoned this as an excuse.
 * CSI New York had a soldier-wannabe who went off his meds and became paranoid that America "wasn't ready" for a terrorist attack. So what does he do? He plants bombs and blows people up, while playing Criminal Mind Games with Mac and the cops.
 * In season seven,  from 24 displays this trope. To the point where he actively kills innocent people, and the FBI agents trying to find him, all so he can have revenge.
 * takes this trope Up to Eleven in the second half of the eighth season, when he gets his hands on the man who.
 * The re-imagined Battlestar Galactica saw the Resistance on New Caprica using suicide bombers against the Cylon occupation force. Colonel Tigh gives us this quote.

"Jane: I have spent enough time with that creep. Staring into the abyss--you know, it's not healthy."
 * A patient, Curtis Ames, from ER, was a good man who crumbled under the loss of his right arm, the divorce of his wife, his children calling another man "dad", and losing his job. He sought to get even with Kovac, who had treated him.
 * Dark Shadows has Reverend Trask, a self-styled witch hunter who had undoubtedly killed many innocent women. As a ghost, he's finally talked into a Heel Face Turn thanks to the opportunity to finally destroy a real witch.
 * Lost: considering who the series's ultimate Big Bad is, this can be inferred as the reason for much of the Others's villainous behavior.
 * On NCIS, Eli David, Ziva's father, is pretty obviously this. Though not a Complete Monster he has crossed the Moral Event Horizon a number of times. And it is obvious that he does so because of his determination to protect his people against vicious enemies.
 * Jenny Shepherd is this about Rene Benoit.
 * Criminal Minds:
 * The actual Nietzsche quote is used twice, once in the first episode, and once in the hundredth episode. It's referenced in the season four finale during the finale voiceover ("How many more times will [my team] be able to look into the abyss"). However, the BAU doesn't really fit this trope, and, in the hundredth episode, . However, Gideon's departure from the team is due to his fear and realization that he's been staring into the abyss for too long and can no longer see humanity past it. He leaves to wander the world for a while and restore his faith in humanity.
 * The Season 14 episode "Luke" has a particularly clear example of this. The unsub is revealed to be
 * The Mentalist: Patrick Jane is so much this when it comes to Red John. The show even goes so far as to hang a lampshade on it in the season 3 episode, "Red Moon:"


 * The only real difference between the way the two use people is that Red John's manipulations end in murder, whereas Jane's tend to end in arrests...but also frequently destroyed relationships, families, and psyches. Geez.
 * Until season 4. Jane, frustrated by his inability to plausibly stop a local serial killer,  When a cult leader calls Jane out on all this, going so far as to tell him he's "coming along," Jane flatly denies it. And continues paddling along a river in Egypt.
 * In Life on Mars, spends much of his career as a copper watching his nemesis become rich through illegal means while he only gets a comparatively paltry wage. To make up for this,
 * This happened to Jim Lahey in Trailer Park Boys. He was driven to great depths of depravity in his effort to save his home from the villainous machinations of Ricky and Julian. Truly, those two criminals were the shit-abyss Lahey looked into and never quite got out of.
 * Veronica Mars implies that Keith and Veronica's career choices are starting to take their toll on the characters's well being and sense of morality.
 * In Community, this happens to Abed when he is taking down a number of Alpha Bitches who humiliate other women. He ends up indiscriminately pointing out the flaws of everyone.
 * Antonia of True Blood was a witch who was raped and murdered by vampires in the middle ages. When she comes back as a spirit, the next logical step is to attempt genocide against the entire vampire race, attacking and imprisoning everyone that stands in her way.
 * King Uther in Merlin is a great example of this. He lashes out at the death of Ygraine due to the magic used to conceive Arthur, and launches into the Great Purge, killing everyone in Camelot even suspected of using magic, and forever banning magic in the kingdom.
 * Hmm—except he invented most of the 'monsters' in his grief, and his own Heel behavior isn't a mirroring of anything he was fighting. Some of his opponents might apply...it's not even clear how much of a bitch Nimue was before he went Rambo on her religion and all its adherents.
 * Person of Interest: Reese is a very self-aware version.
 * Robot Chicken does an extrapolation of Revenge of the Nerds and points out how what the Nerds do is worse that what was done to them, and that what they did was in fact illegal.
 * Space: Above and Beyond gives us Col. Ray Butts, who apparently was born mean and became meaner from being a Marine lifer—he's racist against In Vitros, picks pointless fights with the Wildcards, antagonizes McQueen by taking the squad away from him for a mission he won't explain to anyone, and changes mission parameters mid-mission, again without any sort of explanation. It gets to the point where the squad briefly wonders if he might have killed his previous squad members when they fight a dead marine's body on the planet.
 * Cheese in the Trap: To protect himself from those seeking to exploit him for being a rich man's son, Jung becomes a Manipulative Bastard whose actions also alienate and hurt those close to him.

Music
"In a soldier's stance, I aimed my hand At the mongrel dogs who teach ''Fearing not I'd become my enemy In the instant that I preach"
 * Bob Dylan's "My Back Pages":

"They say that what you mock Will surely overtake you ''And you become a monster So the monster will not break you"
 * He should have been fearful, as is evident to any fan who's heard Slow Train Coming.
 * U2's "Peace on Earth":

"''If you stare into the dark, the dark will stare back back into your SOOOOOOOOOUUUUUUUUUUUUUL"
 * Radiohead's "Bangers and Mash":

Tabletop Games

 * In Warhammer 40,000, The Nighthaunter's backstory is made of this trope. Since he's the grimdark version of Batman, what can you expect?
 * Also: Radical Inquisitors, Commander Farsight some of the time...40K is the only game where even the monsters can succumb to this trope.
 * The Imperium of Man itself can be this. It's a theocratic Dystopia that goes against the secular, rationalist utopia that their Emperor envisioned (The Emperor's Imperial Truth that promotes science and reason is now condemned as heresy, in his name). Their Inquisition is apparently a power-hungry madhouse that is able to order the deaths of millions with a simple "just to be sure that there's no Chaos on the planet". Their armies are known for spreading death and destruction even to those they're helping, and will even fight among themselves if the situation calls for it. As a quote goes, "We used to pray for the Emperor to send us his angels. Now, we pray that he never does again."
 * Inquisitors must be sure that Exterminatus is the best and/or only option, as being baited by the Ruinous Powers into destroying your own worlds is, in fact, seen as just as bad as the world falling to Chaos. Indeed, there's at least one canon example of a Chaos worshipper arranging for an Exterminatus on purpose so that he could dedicate the death of the planet and everything on it to the Dark Gods, an act for which he was elevated to Daemonhood.
 * When you gaze into the warp, the warp also gazes back into you...and laughs. "There is no peace amongst the stars, only an eternity of carnage and slaughter, and the laughter of thirsting gods."
 * Any inquisitor will eventually become this. There are two main sects of inquisitors, the Puritans and Radicals. Puritans believe that they must only destroy Demons, never consort with them or use their wares (even a bolter touched by a demon is considered tainted and must be destroyed). Radicals believe that they are allowed to use demons against other demons, so long as the purpose is pure. These two sects often have clashes and many of the Ordos are divided between these sects. However, all inquisitors start as Puritans, while almost all inquisitors die Radicals. This is because the Wide-Eyed Idealist (and that's saying something, given the setting and how they are willing to behave) that goes into the organisation slowly sees the futility of fighting alone, and slowly rationalize using demons and demonic weaponry against them. Most inquisitors are likewise more likely killed by their own peers than the demons of the warp, usually by trial and execution at having found to harbor demonic artifacts.
 * Forgotten Realms has an elven god, Shevarash, dedicated to nothing more than hatred and vengeance to Lolth and drow (though he readily beats anything Exclusively Evil in the absence of drow targets). Aside of other traits "endearing" to Seldarine and other "good guys", he's a little too close to the main, local, dark deity who makes Lolth look like a nice and caring mom by contrast. Elves who aren't immediately endangered and don't desire vengeance aren't too enthusiastic about his followers either. "The holy symbol of the faith is a broken arrowshaft that has been dipped in drow blood and blessed by a priest of Shevarash." (Demihuman Deities)
 * Another aspect is that while Shevarash himself considers other Underdark threats and theoretically is at peace with Eilistraee, his followers, obviously, seek vengeance for personal reasons—and they swear to "never smile until the last drow is dead" without getting into more specifics. There even was short story Necessary Sacrifices (in Realms of the Elves) about these fine People going over the top even in the eyes of a fresh convert.
 * Old World of Darkness and New World of Darkness:
 * Hunter: The Vigil has this as an explicit risk of taking up the Vigil. Too many hunters have broken in the face of the world's supernatural threats and gone insane, becoming either maniacal killers or, worse, Slashers. The fiction for Slasher makes this explicit, as one story focuses on a hunter who started tracking down and killing mages because one of them killed his buddy; by the time he's working his way down to the relatives of mages, however, another group of hunters tracks him down and kills him.
 * Similarly, in Hunter: The Reckoning, Hunters with very high Virtue ratings may become obsessed with that Virtue. Zealots (including Avengers, Defenders, and Judges) are particularly likely to become Ax Crazy and tolerant of collateral damage.
 * One of the subtler risks in Changeling: The Lost, for many reasons considered the bleakest of the New World of Darkness games. As a Changeling's Wyrd (power meter, roughly akin to, say, Blood Potency) goes up, they remember their time in Arcadia more and more clearly. This is, in and of itself, frightening, but not dangerous. If their Clarity hits zero, though...
 * Mage: The Ascension:
 * One of the drives of the Euthanatos tradition is to put down evildoers and those who disrupt the Wheel of life and death, but going too far risks getting tainted with Jhor and becoming accepting of collateral damage and other extreme methods.
 * The Technocratic Union started out as the heroic Order of Reason, battling the Sorcerous Overlord s oppressing the common folk. Around Victorian times they "reformed" into the modern Technocracy and are now the ones ruling the world from the shadows and stomping out dissent.
 * Beast: the Primordial: Beasts are not good people. They exist to "teach lessons" that more often than not are physically and psychologically ruinous in a way that has uncomfortable undertones of abuse apologism, have no innate checks and balances to limit their "pupils" to the truly deserving, and all their endgames involve fully rejecting their humanity in favour of the monster. However, too many of the Heroes opposing them are not small-h heroes, but rather arrogant, narcissistic psychopaths so obsessed with slaying monsters that they are willing to send allies to die if that helps get them closer to bringing down a Beast. Some have been mistaken for Slashers as a result.
 * In Legend of the Five Rings, the Kuni Witch Hunters are the people who hunt down people who have become tainted by oni/the Shadowlands. In search of these people and in search of how to destroy the Shadowlands, they frequently become tainted and summon Oni.
 * In the prologue of the Dungeons & Dragons book Fiendish Codex II, this is how Asmodeus transformed from a powerful angel to the ruler of all the devils. He started by fighting off the demon hordes, and began to willingly take on the traits of demons to better fight them. Although it could be argued that he was always evil from the start and the changes may have been solely physical in nature.
 * The githyanki from Dungeons & Dragons are so driven to exterminate their former slavemasters, the illithids, and prevent anyone else from gaining power over them that they have become brutal conquerors. The other races are actually more afraid of the githyanki than they are of the illithids, since the illithids's power base is fairly limited. The githyanki even employ similar Psychic Powers. The gith who realized that this path would lead to He Who Fights Monsters broke away from the others long ago and became the more philosophical githzerai.
 * Inverted with Dr. Rudolph van Richten of Ravenloft, who started out his monster-hunting career determined to avenge his wife and son by any means necessary, but came to recognize the wrongness of this after he'd.

Theater

 * The Revenge Tragedy genre, which began in ancient Greece, and was also especially popular in Elizabethan and Jacobean England, is fueled on this trope:
 * Aeschylus's Oresteia trilogy.
 * Hamlet is the most well-known play in the genre (or, you know, ever)
 * The Revenger's Tragedy starts when the Duke propositions Vindice's wife, and when she refuses, he kills her. And rapes her corpse.
 * And what Vindice does, in the course of getting his revenge...is actually a hundred times worse.
 * Titus Andronicus. Another Shakespearean side-splitter.
 * Seymour, the shmuck who feeds the man eating plant in Little Shop of Horrors, was actually the monster himself. All the plant did was sit there and tell him what to do, until Seymour broke his promise, at which point it repossessed Audrey.
 * Sweeney Todd: The Demon Barber of Fleet Street is like the film example...only worse.
 * Cyrano De Bergerac: when Cyrano gives us his Bully Hunter speech in Act I Scene IV, he has just bullied a poor Bore who only slightly bothered Cyrano...So Cyrano, according to the Law of Disproportionate Response, invokes the Berserk Button of his nose so he can dispense Disproportionate Retribution, kicking the Bore’s ass. It’s obvious that Cyrano has been bullied before because of his enormous nose, but he is so Badass compared to anyone else in the play that now he is the bully.
 * In The Secret of Sherlock Holmes, the titular secret is that

Video Games
""Heh, they're all mine... it's been a long time since I had a job this big.""
 * King Kashue will quote this word for word to the Trestkon towards the end of the WorldCorp ending in The Nameless Mod.
 * 2027: One of the possible outcomes in the epilouge is that the Judicians end up becoming those whom they have fought.
 * Ryu from Street Fighter. In the Alpha series and beyond, this trope is a problem that he is constantly wrestling with. His fighting style has an inherent dark side called the Satsui no Hado (Surge of Murderous Intent) that he tries to contain, while his nemesis, Akuma, repeatedly tries to force Ryu to give in to it.
 * It's probably appropriate, given that they turn into monsters, that the Harmonixers of Shadow Hearts face this trope. The downside to their Mega Manning is that in addition to taking in the powers of those they slay, they also take in their hatred, sorrow, and anger. This can have consequences including death, madness and souls being twisted to evil, or becoming feral beasts that exist only to destroy.
 * Warcraft
 * Prince Arthas in Warcraft III. This was apparently engineered (or at least taken advantage of) by the Big Bad, to the point of sacrificing Arthas's target so that the fall could be complete.
 * The box for Wrath of the Lich King even reads, "If you stare long into the abyss... the abyss stares back into you."
 * The Warcraft series uses this one a lot. The only villains in the series (including the Big Bad, Sargeras) that haven't been driven mad by endlessly fighting monsters are the Lovecraft-inspired Old Gods, the demons Sargeras corrupted and forced into his Burning Legion, and the Naga, who were made by the Old Gods (they did not even have a choice!).
 * Clearly lampshade-hung by Malfurion regarding Maiev in Warcraft 3, when he says that she has become "vengeance itself" and hopes that, in her pursuit of Illidan, she will not wreak even more havoc than him. But by the end game of the Burning Crusade expansion, Maiev says that she is 'indeed nothing' after downing Illidan.
 * Illidan himself qualifies after consuming demonic powers to fight the Burning Legion. After being banished by his brother, he briefly works for the Legion in bringing down the Lich King. Failing that, he retreats to Outland to escape their wrath. The evil magic he consumed, which also fills the very air of Outland, drives him mad between Warcraft III and World of Warcraft.
 * After the death of the Lich King, seems hell-bent on becoming the leader of a new Scourge by using the displaced Val'kyr to raise the dead to repopulate the Forsaken. Even Garrosh points out how close  is to becoming like the Lich King. As of the Shadowlands expansion, there is no longer any doubt that this is  goal.
 * The Scarlet Crusade, the fanatical undead-hating organization, which has made the exact same journey as the Lich King/Arthas, have, as of Cataclysm,
 * Same can be said for the renegade group of Scarlet Crusaders that Alliance players assist. They succeed in purging the Scarlet Monastery of its corrupted members but ultimately succumb to insanity after killing their own men.
 * Fandral Staghelm seems to have come full circle on this.
 * This is the biggest risk in becoming a Demon Hunter. Illidan and other villainous Demon Hunters have clearly become drunk with the Burning Legion's power and are often as destructive as they are in their attempts to fight them.
 * The opening quote of the beginning cut scene of Baldur's Gate.
 * And Diablo.
 * And Too Human, which even gets its title from another Nietzsche work, Human, All Too Human.
 * And Legends of Valour.
 * In Puzzle Quest, the main character falls into this in the bad ending, effectively replacing the villain of the story.
 * The video game Phantom Brave features a character like this. His name is Sprout, and he knew that he was absorbing the Big Bad the more and more pieces of it he killed. He thought that he could control it, and when he couldn't, he killed himself and the parts of the Big Bad that were fusing with him. This Heroic Sacrifice weakened the Big Bad for the heroes to fight.
 * In Knights of the Old Republic, we are told that this is what happened to the Jedi Knights Revan and Malak during the Mandalorian Wars prior to the first game. Kreia explicitly states as much in the second game: "As Revan and Malak fought the Mandalorians in battle after battle, they grew to despise weakness, just as the Mandalorians did. In the end, the Mandalorians had taught them through conflict. Shaped the Jedi."
 * We also get a bit of Foreshadowing on Bastila when she's in her full on Knight Templar mode, talking about how a Jedi must "harden their hearts" and "do what is necessary" to fight the Order's enemies.
 * In Final Fantasy IV, the elder of Mysidia uses this phrase to warn Cecil that he could fall into this trap until he willingly sheds his darkness.
 * Nippon Ichi occasionally does this straight (particularly with ), but is more often likely to apply this trope literally—several characters have wound up as demons simply by killing a lot of them.
 * Special mention goes to Prier of La Pucelle,
 * Baby Bonnie Hood/Bulleta from Darkstalkers let her heart become so blackened in her quest to kill all the paranormal creatures that the Big Bad of the third game thought that she was a powerful Darkstalker (i.e Demon) and summoned her to Majigen to fight in his tournament. She failed to notice this, though, and just said:


 * The official sources say that she was actually born with a dark soul which makes her resemble Darkstalkers.
 * In Terminal Reality's awesome monster-hunting game Nocturne, the protagonist, the Stranger, straddles the line of this trope without really going over it. He has an intense hatred of literal monsters, but he will begrudgingly work with them when the situation calls for it, such as by getting inside information from the reanimated mobster Icepick or working side-by-side with the dhampir Spookhouse agent Svetlana Lupescu. On the other hand, the retired Spookhouse agent Hamilton Killian vaults over this trope at a running pace with a pole: when his wife was infected by a vampire, causing both herself and their unborn child to become undead, Hamilton, already a monster killing machine, went completely over the edge, unable to tolerate the presence of even ostensibly good ?monsters?. In the game, he sinks to the point of
 * Parodied in Left 4 Dead: the graffiti in one safehouse reads "WE ARE THE MONSTERS" in large letters, and has a few scathing replies underneath it, including: "No, that would be the zombies", "Have you even looked OUTSIDE", and "I hope you are dead now."
 * "I miss the internet."
 * In Fallout 3, a costumed male character calling himself "The Mechanist" creates a robot army to defend the town of Canterbury Commons from the "AntAgonizer", a costumed female character leading an army of giant mutant ants. The inhabitants of Canterbury Commons tell the player character that the Mechanist's laser-wielding robots are more of a threat to the town than the ant army ever was.
 * Although that's more because the AntAgonizer is a laughable Harmless Villain that the townsfolk can deal with just fine by themselves, rather than any real fault on the Mechanist's part (well, he is a delusional "superhero", but what costumed crimefighter isn't?). If Canterbury Commons was up against Raiders or Enclave, the Mechanist's ability to crank out gatling laser-wielding robot soldiers would probably be better appreciated.
 * In Penumbra: Black Plague, after learning a big lesson about the Tuurngait being  and completing some trials to show that   you contact the outside world and   Then again, we never get proof that they aren't lying...
 * The Church in Tsukihime is known as being fanatically zealous about killing non humans. Other characters imply, and we have even seen, that there are some demon hybrids/vampires who are neutral or good. The Church does not care, so it does not tend to get along with those like the Tohnos (hybrids, but avoid inversion), Arcueid (vampire, but kills vampires and is too tough for them to beat), and the Nanayas, who only targeted the Inverted. Notably, the only Church characters we see are a bit heretical for not blowing everything up first and then covering it up.
 * It's not surprising for them to be heretics considering that the Church is repeatedly stated to have no influence in Japan—since any members of the Church are the only representatives of it for dozens, if not hundreds, of miles, it's pretty safe to act as they please.
 * Blackwatch from Prototype is about as evil as they come, fighting a merciless viral threat that doesn't care for the laws of war or human rights. Conversely, our player character Alex Mercer starts off real sociopathic and very slowly develops a conscience.
 * Mostly, it is implied, by consuming people who have one.
 * Ace Attorney's definitely qualifies. Starting as a   While he was successful, he admits that he acted out of revenge rather than the desire for justice.
 * Even  himself becomes this in Apollo Justice. After having his attorney's badge taken away for
 * In Shadow of the Colossus, you . Granted, later on, you
 * Tales of Vesperia. The guild-wide Freudian Excuse doesn't excuse the Hunting Blades from acting worse than the "monsters" they hate so much.
 * Yuri himself skirts this trope and is well aware that he almost falls into this. Ironically the one who did fall into this is
 * In Soul Calibur IV, Maxi has let his hatred of Asteroth consume him to such a degree that he's willing to use the soul-consuming sword his friends have been desperately trying to destroy to fight him.
 * By the time of Mega Man Zero, Mega Man X admitted to no longer caring about the enemies he fought. What's worse was that if it weren't for the Continuity Snarl between the two series, he X, however, was savey enough to realize this and took himself out of the fight   before he could go over the edge.
 * Dr. Weil is a straighter example. For all the acts of revenge he pulls on both humanity and the Reploids, he completely ignores the fact that he has become what he hates the most: a Maverick, in all definitions of the word in that universe.
 * In Mega Man ZX, the manner in which the Artifact of Doom, Model W, sows conflict and destruction in the world - and in doing so, grows stronger - exists at the point of intersection of this trope with The Virus. If fighting monsters (both literal and metaphorical) ultimately causes you to become a monster - with Model W accelerating the process - then those who fight you will also become monsters; apply recursively. This never-ending cycle is explicitly pointed out late in the second game. Appropriate, given who Model W used to be...
 * Shadow Complex flirts with this in the subtext. The main character notes that killing has become easier for him, and he even gains a suit of the very same Power Armor the enemies use, making him identical in appearance to them.
 * Mass Effect has Garrus Vakarian do this. He originally quit C-Sec to join Shepard's team because of all the red tape, as he says that it shouldn't matter how he got the job done as long as he did it. In the first game, his personal mission involves finding an organ-harvesting doctor who got away from him - Garrus's first instinct is to kill him on the spot ("I'd harvest your organs first, but we don't have the time.") Shepard can either encourage him or convince him to at least try and take him alive.
 * In the sequel, his loyalty mission has him tracking down the only member of his vigilante squad to live through an ambush (considering that he was The Mole), wanting to kill him. Shepard has the option to cooperate or block Garrus's shot and have the guy tell him that he's a dead man walking.
 * Liara almost does this as well as her quest for revenge on the Shadow Broker for trying to hurt Shepard causes her to turn from a sweet and innocent archaeologist to a ruthless information broker who barely trusts anyone and threatens people in the same way her mind-controlled mother did two years before. She even alienates Shepard, the one person she risked everything for, when s/he finally comes back from the dead. Ultimately she eventually averts this...kind of.
 * The Reapers have this as one of their special powers. Indoctrination is an effect that slowly alters the minds of everyone in their vicinity to be more compliant towards their suggestions and assume their way of thinking. This effect even persists if they are dead, making even studying their remains dangerous. It seems very likely that
 * Suikoden Tierkreis makes this recursive. You can take down the Big Bad the easy way by sacrificing the lives of all your allies, or look for another, harder way to beat him.
 * In Dragon Age, the Grey Wardens are an order of warriors, rogues, and mages dedicated to fighting the darkspawn. To do so, they ingest darkspawn blood, making them more powerful and allowing them to sense the darkspawn. This also slowly turns them into darkspawn like creatures. The Wardens are also known for their ruthlessness in fighting the darkspawn, as they have no qualms against killing people who have been infected with the darkspawn taint, although they are nowhere near as brutal as the darkspawn.
 * Loghain Mac Tir is the main antagonist of the game, and is so paranoid that he refuses to ask for help when Ferelden is on the verge of defeat, and sparks a civil war, simply because the king was going to ask for help from the Orlesian Empire, which had invaded Ferelden years ago. As such, he leaves the king to die in the opening battle, because he's paranoid that it's handing Ferelden back over to the Orlesian Empire, when the entire country is about to be destroyed. In the process of this all, he becomes everything that he hates, and never realizes it—some Ferelden citizens will speak in hushed tones that he now resembles the very Orlesians he once threw out of the country.
 * In order to fight the darkspawn, the dwarven smith Caridin created an Anvil that allowed him to
 * Branka—was she a monster before, or did she become one due to her obsession with the Anvil? The epilogue shows that anyone who gets hold of the Anvil becomes pretty inhuman (indwarven?) with it's use, so Caridin has a good point. But Branka went that extra step and mutated her own people, who hunted monsters with her, involuntarily into monsters.
 * In Dragon Age II, Anders has a very literal case of this as he has voluntarily allowed the spirit of Justice from Awakening to possess him, but his anger has warped Justice into a force of vengeance against the Templars, and Anders has become very much a Well-Intentioned Extremist.
 * To be honest, the entire storyline is built on this. It really fuels the game's relentless Grey and Gray Morality.
 * Considering the back story of Killzone, the Helghast could be seen as these. Fleeing to a Death World after losing a war trying to resist the UCN's tyranny, they became as bad as the very people they fought.
 * Andrew Ryan of Bioshock could be seen as this. He built Rapture in an attempt to create a free-market paradise where the world's greatest entrepreneurs, scientists and artists could live free of government regulation, taxation and censorship. Yet both the flaws in his society, the actions of Frank Fontaine and the invention of ADAM have caused him to become a brutal fascist dictator by assassinating dissenters, instituting curfews and executions, nationalizing Fontaine's business and finally using pheromones to Mind Control Rapture's mutated populace, in short not only becoming a "Big Government" much worse than the one he founded Rapture to get away from but even destroying free will, the one thing he considers that truly "separates a man from a slave".
 * In BlazBlue, Kokonoe has a seething hatred of Terumi Yuuki. In her attempts to bring him down, though, she has committed some acts that are earning her the ire of the fanbase, like treating Lambda-11 as if she's nothing but an inanimate tool. She is aware of this fact (doesn't even try to cover it up with justifications) and notes to herself that she has crossed the point of no return a long time ago.
 * Maxim from Lufia II starts off as this trope, but quickly evolves into a more standard hero.
 * Norman Jayden from Heavy Rain qualifies. He is competing with Carter Blake because he called his methods useless. For example, Jayden will go from beating Blake for beating Ethan to threatening Mad Jack that he'll kill him if he doesn't answer his questions.
 * This is debatable, as it is more up to the players choices as to how Jayden will act, and it is more than likely he was simply bluffing Mad Jack.
 * Jayden is also visibly shaken from almost being killed by the guy, and suffering from withdrawals to boot.
 * The Patriots in the Metal Gear Solid series is a good example of such an issue: They were initially formed to essentially eliminate the Philosophers and replace it with The Boss's teachings. Unfortunately, soon after that, Zero, Paramedic, and Sigint ended up becoming corrupt, and eventually became the very thing that they attempted to remove.
 * The first preview of the Demon Hunter class from Diablo III seems to be this trope wearing a hood.
 * Tekken 6's Jin Kazama quotes Nietzche word for word after his final encounter with Lars. As far as Jin is concerned, however, he will do whatever it takes to end his bloodline, including becoming just as evil/hated as Azazel, who was responsible for most of the conflict in the Mishima bloodline.
 * City of Heroes allows the player to fall from a Hero to a Vigilante by taking some morally dubious actions, and eventually fall to Villain. Fortunately, you can pull a Heel Face Turn.
 * Subverted in Touhou 12 with the origin story of Byakuren Hijiri, a youkai hunter who came to genuinely sympathize with youkai and eventually converted herself into one. In the same game, Sanae Kochiya is warned that if she continues to hunt youkai with such zeal, she will play this trope straight. There has been mention of Marisa Kirisame possibly converting herself into a youkai in a bid for power, though it hasn't happened.
 * Happened to  during the Elder Wars in Lusternia. They start out employing methods that the other Elders found questionable, and end up Jumping Off the Slippery Slope and eating their fellow Elders to fuel the war effort - which happens to be the same threat presented by their foes, The Soulless.
 * A possible interpretation of  in Chrono Trigger.
 * It's lightly hinted at in Kingdom Hearts II that Sora is slowly falling into this, based on how much more aggressive, ruthless, and rude he is toward the enemies than he was in the first game. Then again, fighting the forces of Darkness themselves can take its toll on people, especially when you have.
 * Word of God says this outright, also stating that Anti-Form is a product of it.
 * DiZ/, who, in his efforts to make sure that neither The Heartless nor the Organization XIII takes over the universe, rely on kidnapping, psychological manipulation, and playing the You Have Outlived Your Usefulness card on his Nobody allies. At least he regrets it in the end before he
 * In NieR, the protagonist develops an all-consuming hatred for the Shades after The Shadowlord kidnaps his daughter at the end of the first half of the game. It's really shown in the New Game+ where
 * God of War's Kratos. What started as a quest to rid himself of the nightmares turned into a festival of divine slaughter. By the end of the third game, he's arguably become worse than the Gods he despised so much.
 * Honkai Impact 3rd: Whatever just cause Sirin might have had in seeking vengeance on those who experimented on her, she lost any claim to the moral high ground the moment she stopped restricting herself to her tormentors and started indiscriminately bombarding civilians.
 * in Lost Judgment. However justifiable one might consider killing bullies who drove their victims to suicide, by the time he's blackmailing bullies with their wrongs so as to coerce them into helping him and getting innocents caught in the crossfire, it's a lot more murky as to whether it's still long-overdue justice he's meting out.

Web Comics
"Gil: The worst of it is, it sounds so horribly close to whatever it is Lucrezia did to Agatha - and the last thing I need is for it to get out that the Empire has stooped to a version of the Other's mind control!"
 * Jenn of Casey and Andy just manages to catch herself doing this.
 * Dominic Deegan has a literal version of this with Karnak.
 * Professor Broadshoulders from Zebra Girl is obsessed with destroying demons and people tainted by demons, to the point where he sacrifices his very soul, giving into his own demonic curse and physically transforming into a demon, to destroy Sandra, the titular zebra girl, despite the fact that Sandra was still a good person, despite being transformed into a demon herself. Appropriately, it was Broadshoulders's attempts at destroying Sandra that finally pushed her over the edge, turning her away from wanting to cure her condition to indulge her demonic hunger for pain and torment.
 * from The Order of the Stick became a very literal example of this trope but has since "recovered" and now regrets their actions.
 * And Redcloak, depending on your interpretation. Although in his case, it's less a matter of fighting with (against) his enemies and more fighting with (alongside) Xykon.
 * This is invoked in Gastrophobia: when Bambikles seeks to avenge his mother by killing the monster who took her life, he's told that "To kill a monster, you must become a monster." So he does.
 * Arguably, Baron Klaus Wulfenbach from Girl Genius, if not personifying this trope, certainly straddles the line. He is referenced on many occasions as being capable of acting very ruthlessly and, if need be, wiping out whole cities, but does it because the previous anarchy of the Long War between the Sparks of Europa was evidently worse.
 * Of course, he was quite unhinged about anyone and anything that could have anything to do with whatever had anything to do with Lucrezia... so eventually he ran out of the slippery slope.

""I never understood what's supposed to be so bad about that... I mean, I'd see that the abyss is deep, and the abyss'd see that I'm shallow - but it's not like I've been denying that, anyway...""
 * As is usual, Penny Arcade gives us quite the atypical example.
 * The Vatican and Aesir churches of Cry Havoc level cities to destroy a handful of daemons. Although, seeing as the last time their foes congregated, a decade long war that killed half of the worlds population occurred, they may be more justified than most.
 * Amazing Super Powers provides an amusing variation of this.
 * In The Adventures of Dr. McNinja, when the doc is subject to More Than Mind Control by an evil motorcycle helping him hunt down King Radical, this trope provides the "More Than" part.
 * When the second part of the phrase (the abyss part) was used in The B-Movie Comic, creator Roman Wunderlich declared in The Rant that he didn't understand what the big deal was:

"Sora: (From down in the abyss) Hi Riku! Riku: Damnit Sora! Get out of the %$#@^& abyss!"
 * Parodied in Ansem Retort.


 * Bittersweet Candy Bowl: After years of taking Tsundere-ish abuse from Lucy, Mike finally shall we say, "gets her back" by not only . The problem is, which makes him as bad, if not WORSE to her than she ever was to him.

Web Original

 * In The Gamers Alliance, Refan becomes a more jaded and bloodthirsty killer as he ends up having to fight more and more enemies to protect his loved ones. When the enemies get tougher, he has to resort to using his demonic side, which is slowly corrupting him.
 * The Dove, from the Global Guardians PBEM Universe, started out as a standard street-level superhero who concentrated on finding and stopping serial killers. Ten years later, after he is arrested for the murder of his latest target, he suffers a Heroic BSOD when it is pointed out, finally, that he isn't a hero but rather is just a serial killer himself...one who targets other serial killers.
 * Adam Dodd in Survival of the Fittest turns into one of these for a good while during his tenure on the v1 island. His obsession with getting revenge on Cody Jenson leads him to mow down a good six or seven of his fellow students, despite his supposedly heroic motives. In something of a subversion, however, he lives to come to realise that his actions have been misguided and returns to a more conventional Anti-Hero mold.
 * In Sailor Nothing, Himei worries that she is turning into this.
 * Satirized by The Onion, "Little Boy Heroically Shoots, Mutilates Burglar".
 * Anna Dollerious in an installment of The Secret Life of Dolls, which was named after the trope itself.
 * The Union series. Combined Forces - Team 4 falls under this trope. While they start off as idealistic as soldiers go, they eventually devolve into bitter reflections of themselves that kill because they can, not because they have to. Taken to the extreme with Shadow Agents, clones born and bred to go as far into this trope as possible, becoming little better than what they're fighting.
 * Moviebob made a reference to this trope when he was describing the progression of the Shrek movies, pointing out that while the first movie was original and entertaining, the last two became that which they had been mocking in the first movie.
 * Equestria Chronicles takes this trope and dances all over the place with it.

Western Animation

 * Depth Charge in Transformers: Beast Wars snubbed most of the Maximal cast to continue his hunt for Rampage, who had murdered the colony he was supposed to protect beforehand. The one time he decided not to put his vendetta before an important mission, he was ambushed by his prey, and chose to finish him off in a suicide attack rather than return to his duty. To be fair, Rampage didn't give him much of a choice in the matter. Rampage was purposefully standing between Depth Charge and his mission to stop the Nemesis. A suicide attack might've been overkill, but Rampage wasn't going to stop and just let Depth Charge go after Megatron.
 * Being frequently AWOL allowed him to pull off several Big Damn Heroes moments, but that doesn't excuse his not being there fighting in the first place. He also utters this trope's Stock Phrase word-for-word: "It's not revenge I'm looking for, it's justice."
 * In his second appearance, he was busy fighting with Rampage while the Maximals carried out a very important mission regarding the retrieval of the security system from their old base. After his fight with Rampage, he decided to check up on the Maximals and see if he could help, but ended up causing a blunder (thanks to the lack of communication) that lost the system to the Predacons.
 * Depth Charge even went head-to-head with Optimus over it, when he showed no remorse at Cheetor's death and didn't care about what really happened to him. Optimus-fucking-Primal had to threaten him with a lobotomy to get an answer and some respect. Once Depth Charge opened up a bit, he revealed a fondness for Cheetor.
 * To be fair, Depth Charge's lack of respect was shown after Cheetor had been revealed to be alive but before it was known what was happening to him (a painful transition into becoming a Transmetal 2). After being called in to answer questions, Depth Charge (not knowing that Cheetor had gone AWOL) told Optimus to "ask the cat".
 * Let's also throw in the Maximal government; they locked down all information and locations related to the Great War, Big Brother style, they experimented on their own citizens in order to copy a condition in a Decepticon's spark that rendered it immortal (resulting in the creation of Rampage and the atrocities he committed, above), and their treatment of the Predacons is very much like the treatment of the Allies towards post-World War I Germany. All in the name of keeping the peace and not letting the atrocities of the past come back to haunt Cybertron again.
 * Megatron himself is this in several continuities, such as most of the comics series, Beast Wars, Transformers Animated and Prime, usually getting his start fighting against an oppressive and unjust Autobot/Maximal regime, and then going more mad and power-hungry than the guys he rebelled against. In Animated, he's fallen less far than usual, to the point where he would be downright antiheroic if not for his rampaging anti-organic bias and somewhat psychopathic minions.
 * General Wade Eiling in Justice League Unlimited became so obsessed with taking down the show's titular League (and metahumans in general), that he injected himself with a Nazi-created Super Soldier serum so he could more readily combat them. His only claim to fame after this? Beating up seven non-powered second-stringers and causing the same superhuman fear-mongering from the citizens that he swore to protect from the League. He did put a Lampshade Hanging on this trope at the end.
 * Hama of Avatar: The Last Airbender.
 * Ditto Jet, for that matter, although he calmed down a little eventually. And by "eventually", we mean "too late".
 * Also, that random Earth Kingdom general who was obsessed with forcing Aang into the Avatar State. Fong, I think.
 * Also, Katara in "The Southern Raiders", though she manages to restrain herself at the last minute.
 * This is also an element in the propaganda history the Fire Nation feeds its kids.
 * On ReBoot, little Enzo Matrix never took the presence of Lawful Evil Megabyte too seriously, until Megabyte finally took over Mainframe, exiled The Hero, Bob, into the WWW and pushed Enzo into despair until he was trapped in games. When Enzo came back much older and much more serious, killing Megabyte was all he could think of. There was even an episode where, in a simulation of Mainframe, he was in Megabyte's body and easily acted the part unconsciously. All this anger towards a virus that once gave Enzo a guitar for his birthday.
 * However, giving Enzo a guitar was more a 'Rule of Funny' moment as well as pressure to keep it 'kid friendly' from the meddling executives. Once free from said executives, they were free to make a much darker and less kid friendly story where Megabyte showed his True Colors.
 * Agent Kent Mansley from The Iron Giant, whose fanatical anti-communist efforts made him unable to accept the titular robot's overtures of peace, even when the general he was advising was willing to stand his soldiers down. He then got a nuke launched at a small American coastal town. Where he happened to be.
 * A major theme in the "City of Thieves" episode of Adventure Time, though it is used more for the Rule of Funny.
 * In Gargoyles, Demona and, later on, Jon Canmore/Castaway.
 * Aaahh Real Monsters: Simon the Monster Hunter wanted to prove that monsters were real, and fancied himself the Only Sane Man, despite how the world at large viewed him as a lunatic. As the series progressed, he developed a personal vendetta against Ickis that drove him to increasingly nastier and ever-more-insane plans, eventually becoming a Complete Monster dead-set on getting his revenge on monsters in general and Ickis in particular.
 * South Park loves this trope. In one particular episode, in order to keep tomato Kenny alive, Stan and Kyle enlisted the Republicans, and showed Kenny on national TV like that...only to learn that his last wish, if he were ever hooked up to a feeding tube, was "...for the love of God, don't show me on national TV like that."
 * Kyle gets this most, considering his personal hatred for Cartman (not that Cartman's Complete Monster actions make it any less justifiable).
 * The episode "Butterballs" depicts anti-bullying advocates as being no better that those they are fighting against.
 * The Simpsons played this trope surprisingly straight in the "Revenge Is a Dish Best Served Three Times" segment "Revenge of the Geeks", in which, the Elementary School's group of nerds, tired of being bullied, created a weapon and gave it to Milhouse, since he's the only one with hand-eye coordination. At first, he uses the invention to get back at the bullies, but then he started to use it on anybody who wronged him in the past, to the point of becoming a bully himself. Lisa, who was narrating the story, says that the aesop of the story is that revenge can make you as bad as the people who harm you. Lampshadesd in "Sex, Pies and Idiot Scrapes," where Homer and Ned become bounty hunters and Homer scolds Ned for "not becoming as bad as the people they were hunting."
 * After time travelling to a Bad Future, Gosalyn finds that her absence made Darkwing Duck turn into Darkwarrior Duck whose idea of justice is Disproportionate Retribution. He is, without a doubt, far worse than any of the villains who he has already disposed of permanently.
 * The seeming moral of the animated short "Who's Afraid of Mr. Greedy".
 * From the 2017 reboot of DuckTales, the Phantom Blot. His initial goal was, if not particularly noble, justified, as he sought revenge against Magica DeSpell, who had destroyed his village and killed his family for no reason except For the Evulz. However, the Blot became just as much a monster as she was, willing to kill anyone - even children - who got caught in the crossfire or had what he needed to pursue his goal.

Real Life

 * There are many cases where a corrupt regime is overthrown by rebellious revolutionaries, but later becomes this. One of the leaders of the rebellion becomes the head of state and will use the Revolution as an excuse for anything they do because Utopia Justifies the Means. Unfortunately, since power corrupts, everything will be back to the way it was before, or worse. The Soviet Union under Stalin is one of the most famous examples of this. Both of George Orwell's major works, Animal Farm and 1984 are parables on this happening on how this nigh-inevitable cyclic nature of politics happens, specifically satiring this development in the USSR. This isn't a recent development, either: see The Inquisition, and France under Maximilien Robespierre. For that matter, a few establishments dealing with revolution/terrorism, civilised or not, have turned to Knight Templar methods little better than those they oppose.
 * Winston Churchill may not have actually said "The future fascists will call themselves anti-fascists" despite ironically becoming a bit of a Knight Templar in his desperate effort to retain the empire, but there are some anti-fascist groups such as Unite Against Fascism (UAF) that actually do have such an aversion to fascism that they will actively try to deny fascists from exercising non-offensive free speech, one infamous gaffe being a local UAF branch removing a wreath from a town's cenotaph, which led to a Labour councillor, ex-Mayor, and former soldier strongly reprimanding them for doing so. UAF is a leftist organization which actively fights (sometimes literally so) with/against the people popularly known as the "far right" in the UK. Sometimes, it's hard to tell who's who. They also have links to several Muslim extremist groups, which many people are unaware of. Also not helping matters is the fact is that the sort of groups the UAF oppose bandy about the term "Islamofascist" for anyone who dares to defend the Islamic faith and/or Muslims's right to freedom of religion. UAF and EDL both claim to be anti-racist, pro-free-speech, non-violent, and anti-fascist. Other extremist anti-fascists have been documented using violent protest to force the cancellation of speeches from right-wing figures, confronting seniors, shooting and beating conservatives, and generally doing the whole Nietzschean fighting monsters thing.
 * Dwight D. Eisenhower's farewell speech, urging Americans to guard against the rise of a "military-industrial complex" (famously, the first time such a phrase had been stated publicly) in the struggle against the USSR could be seen as an attempt at averting this trope.
 * This aversion mostly failed, as many changes to the United States government to help defeat communism, such as massive peacetime defense budgets, centralization of most government functions (Commerce, Energy, Education, Interior) at the national level, and a hundredfold increase in the budget for espionage and black ops, made the US much more like its Soviet Counterpart.
 * The human immune system, in the process of destroying intrusive micro-organisms ("fighting monsters"), can get over-sensitive and start reacting to harmless things like dust, causing rashes and eczema, among other allergic reactions of varying severity, that have very real effects on lifestyles. In fact, the human immune system is pretty bad on this account; if the accounts are to be believed, cytokine storms were responsible for quite a lot of the deaths of the influenza victims, instead of the virus itself.
 * Cytokine storms also lead to Systemic Inflammatory Response Syndrome, and the subset of those, septic shock. In either case, the immune system is curing the disease by killing the patient.
 * Autoimmune disorders (most famously, lupus), which is when the immune system views a part of its own body as a foreign invader and attacks it as it would a pathogen or allergen.
 * In Japanese mythology, a person who kills many Youkai will be transformed into a youkai. This occasionally gets played around with in games, manga, and anime from the country—for example, in La Pucelle, this is the basis for a Nonstandard Game Over, one that gets taken more or less as canon in the Disgaea series. And in Inuyasha, the murderous Bankotsu of the Band of Seven manages to transform his weapon into a demonic blade by using it to kill 1000 youkai and 1000 human warlords. It also shows up in Saiyuki, where it's a part of Hakkai's backstory.
 * The Abu Ghraib scandal, in which military prison guards in charge of Iraqi criminals indulged in chillingly gleeful torture in an attempt to break the prisoners' wills. See these analyses for more.
 * The chief cause of Abu Ghraib was a senior NCO reservist who had been a career corrections officer in civilian life, who indulged in prisoner abuse primarily for his own sadistic amusement and encouraged his subordinates to do likewise. It turns out he had a long history of doing such things in the US as a prison guard as well. Which is still a shining example of this trope, a man whose life's work was in keeping brutal criminals locked up and who grew to be far too much like them himself.
 * The early career of Malcolm X. To the point where he said things like, "The white man is by nature a devil and must be destroyed." He backed off of this stance shortly before his murder.
 * Thomas Jefferson. He became president in order to prevent his rival, the federalist Alexander Hamilton, from taking the post instead. For years, he fought strong executives and central governments, and thought standing armies were a threat to liberty. Once he became the executive of a central government, he overstepped his bounds not once, but twice, and issued a coup against a standing government.
 * Jefferson himself is said to have been aware of this inconsistency, and is supposed to have said something like "One cannot be anti-government when one is the government."
 * Many, many gangs started as groups of people that needed protection from other gangs.
 * The Aryan Brotherhood, a white supremacist prison gang that is responsible for lots of murders (including that of wardens) and extremely dangerous.
 * Mara Salvatrucha or MS13. They started off as a group of Salvadoran migrants that needed protection from local gangs like the Crips and Bloods, but eventually started to take part in various crimes themselves. In fact, they are, by now, one of the most dangerous and powerful gangs in the world.
 * Robert Mugabe as President of Zimbabwe, particularly since the early 2000s.
 * Averted by Nelson Mandela after the end of the apartheid rule in South Africa, when he established the Truth and Reconciliation Commission.
 * The French Revolution. The people that instigated it intended to set up a republic modeled on America. Instead, they turned out to be every bit as bad (if not worse!) than the government they replaced. Mass murder was rampant, and, eventually, they just started killing people for the smallest of infractions. This was what paved the way for Napoleon Bonaparte to take power.
 * Even more poignant on a personal level for the central face of the Revolution, Maximilien Robespierre. He started out as an idealistic lawyer opposing the absolute monarchy and the practice of capital punishment. He went on to start off the Reign of Terror, executing hundreds daily as counter-revolutionaries in his increasing paranoia, destroying the ideals of the Revolution in the process. He embodied, perhaps, the most poignant example of Motive Decay, ever.
 * The South African vigilante group PAGAD (People Against Gangsterism and Drugs). Originally starting out killing drug dealers and other unsavoury characters, they eventually resorted to bombing Jewish synagogues and murdering the judges who were taking cases against them, resulting in them being officially declared a terrorist organisation - and their subsequent long-term decline.
 * A common admission/ideation from cops worldwide is how hardened they become to crime that they start to think and act a little like the street scum that they are out there to stop. Okay, certain countries have more...lax rules in how cops go about police work, but the majority of cops, like it or not, start to become bitter and lean towards the more heavy-handed, obnoxious, and generally violent side sooner or later. When all you deal with day in, day out is Crapsack World, and Failure Is the Only Option, you just sort of run with it.
 * The Armed Forces of numerous countries in Latin America during the Cold War (Chile, Argentina, Guatemala, and El Salvador are a few examples) perceived themselves as the 'defenders of Western and Christian civilization' against the 'Marxist cancer' and, therefore, believed that they did what they had to do.
 * In turn, Fidel Castro and his supporters and many other (often left-leaning) revolutionary groups throughout Latin America like the Sandinistas in Nicaragua and the FARC in Colombia took this same attitude. They believed in doing whatever they had to do to fight brutal, corrupt governments and what they perceived as imperialism and the corruptions of capitalism. Unfortunately, both the revolutionaries and the (usually) right-wing armed forces often went WAY TOO FAR.
 * George Orwell wrote of such observations in Allied-occupied Germany following the end of World War II in the essay, 'Revenge Is Sour'.
 * The Chinese initially opened trade relations with America under the notion that "the capitalists will sell us the noose to hang them with", to quote Marx. Today, try to find anything communist about China.
 * To sum it up, China is a faux-Communist nation, with their well-intentioned ideals corrupted by the totalitarian government.
 * Your Mileage May Vary on whether China is "corrupted" by being no longer communist (or probably un-communist at all). Chairman Mao, the founder and dictator of Communist China, caused the deaths of 40,000,000 to 70,000,000, at least four times as many as the Holocaust. Also: whether or not the Chinese communists's ideals were truly well-intentioned at the end of the day.
 * Common in flame wars when politics, religion and/or Trolls are involved. Bad arguments in particular cause rage which cause even more bad arguments.
 * Crips began as a movement to protect the Black community from Police Brutality similar to the Black Panthers and ended up as one of the largest and most infamous gangs in the world.
 * The Taliban formed their base of power around being sane, religious people external to the civil war that had raged for decades, who could bring order and safety. This got them around a third of Afghanistan before the stresses of trying to fight equally powerful coalitions and the temptations of being a bunch of young men with a lot of power and the level of socialization you'd expect from a bunch of semi-war orphans ('talib' is a student of religion; they recruited mainly from the refugee schools over the border in Pakistan) got to them. Not that they were ever really nice, but they were trying to sort things out for the general benefit. Tyranny, as usual, ensued.
 * This is why therapists have to see therapists themselves; because talking to crazy people for a living can take a toll on one's sanity.
 * It isn't unheard of for a student to become a bully after being excessively pick on in school to the point where he fights off his tormentors... only to become this to others.
 * Hollywood got its start because numerous filmmakers moved there in an effort to avoid Thomas Edison's iron grip on film patents and his licensing fees. The MPAA was formed as an effort to push back against government censorship. Nowadays, it's the big movie business that's pushing to strengthen intellectual property enforcement and censorship with proposals such as SOPA and ACTA.
 * Jeff Gerstmann was fired because of the corrupt state of game journalism, and the cause of his firing was brutally censored by his former employer. He went to found his own site, where he went on to found his own site, drawing many viewers for claiming to stand against this corruption. The next decade when another story of journalistic corruption breaks, he's brutally censoring any discussion of it and defending the corrupt.
 * William "Billy D" Usher created the One Angry Gamer website to fight back against the rampant and blatant left-wing political activism that he felt had taken over the gaming news media. When gaming journalism apparently moved to a more centrist position, he then started grasping at the tiniest of straws in order to stay relevant.  He eventually reached the point of manufacturing controversies and essentially becoming a terrible right-wing activist who wrote primarily angry political rants punctuated with only the occasional decent gaming news article once in a blue moon.