Tragic Monster

When a background character becomes a monster, you generally don't care. You just watch the protagonist wipe the floor with them like they're nothing. But what happens when it's a character that The Hero has come to love and care about over the course of the work?

Nothing can wrench the gut of any protagonist like being forced to fight a loved one or ally (except maybe if said loved one is weeping uncontrollably and begging for death as he/she tries to kill you). This isn't just Brainwashed and Crazy, this is forcing the unlucky character to become a terrible beast or other nasty critter with no means of changing them back. Not that it will tend to stop the heroes from trying. In many case the hero will do everything they can to avoid Staking the Loved One. Even if it means keeping them contained for their own safety while they hunt for a cure.

In many works, this can amount to a form of Player Punch. It's par for the course for any Zombie Apocalypse, since doubtless somebody important is going to get turned into a zombie.

May bring Dying as Yourself into play.

If a loved one must kill the monster, see Staking the Loved One.

See also The Virus, Came Back Wrong, Was Once a Man, And Then John Was a Zombie, Resist the Beast, and What Happened to Mommy?. Compare Interrupted Cooldown Hug and "I Know You're in There Somewhere" Fight. For cases when it's only a monster imitating a loved one, that's Shapeshifter Guilt Trip. Sub-Trope of Face Monster Turn.

Anime and Manga

 * Akira, when the main character Kaneda must battle his super-powered and tormented friend Tetsuo, who goes on a destructive rampage after his sanity is nearly split from the disturbing mental images that plague his mind after acquiring said super-powers. Eventually, Tetsuo loses control of his powers and transforms into a truly Nightmare Fuelerrific amoeba like blob of flesh and organs that consumes everything in its path. This is an unusual example in that while Kaneda and Tetsuo are best friends, they actually relish fighting each other (mainly because they're friends).
 * Cyborg 009 has three of them: 0011 (a man who wanted to see his family), 0012 (a woman whose house and memories were manipulated by Black Ghost) and 0013 (who helps the innocent bystanders, punishes criminals, is a Worthy Opponent to Joe and )'
 * Fullmetal Alchemist: All you need to remember for this trope is one name: Nina. She wasn't a vicious monster, thank goodness...but she was living a life of constant pain. On the lighter side of things, Scar puts an end to that pain by killing her. While her mother doesn't have as much screen time, it's also shown that she suffered a similar fate.
 * Witchblade has a very Tear Jerker version of this. Most of the show's monsters are Ex Cons, creatures made with Human Resources. It gets heartbreaking when you realize at least one of them not only retained its own mind, but actually is horrified at what it had become and begs The Hero to take them down before anyone else died.
 * Baldr Force EXE:
 * Priscilla in Claymore. She is affected by Horror Hunger, and remembers her parents getting eaten every time she eats someone. More recently,.
 * Hans, the last of the Flamethrower Troopers from Pumpkin Scissors. He cannot remove his suit because it would kill him like it did his friends and he works for the Silver Wheel because he was promised a way to get it off. Being in that suit for so long, he has lost the ability to feel and has become a machine who only Oland could connect to.
 * in Bleach. Infested by a parasitic Hollow, they have no control over their bodies and can only watch as the Hollow slaughters their friends and subordinates..
 * Sora Inoue, Orihime's older brother. Consumed by loneliness and regrets as years passed, he transformed into a Hollow that sought to destroy everything that caused those feelings of sorrow, namely Orihime and her friends. Even when he temporarily regained control and apologized, he had to die.
 * Generally, a Hollow is this, as they lose control over themselves and become hopelessly violent. Driven mad by sorrow and loneliness, they destroy everything they valued in life and only through the purifying blade of a Shinigami can they find release from that cycle. The exception is Shrieker who is still in control of himself after becoming a Hollow but commits terrible actions anyway.
 * A definite Player Punch for Ichigo. He had never realized until that point that Hollows were once people, and when he finds out it's while trying to stop a spirit who died at his family's clinic.
 * Similarly to Bleach, the human souls of Akuma in D.Gray-man are invisible to everyone but Allen, and they can't stop themselves from killing dozens of people unless someone destroys them. Level 2 demons are intelligent, and they all have a tragic backstory attached.
 * In Puella Magi Madoka Magica, in the main storyline, as does . Given the nature of magic and magical girls in the Puella Magi universe,.

Comic Books

 * The Joker can qualify in some mediums, or at the very least an extremely dark Woobie, Destroyer of Worlds.
 * The Incredible Hulk is a subversion. When Bruce Banner first becomes the Hulk, he plays this straight, as he simply won't stop destroying things. However, the Hulk goes from being a destructive monster to actually being heroic if still destructive and actually manages to control his destructive rage, so compared to other examples of this trope, he got the good end of the stick.

Film

 * Jeff Goldblum's portrayal of the doomed Dr Seth Brundle in The Fly. Much as he tries to hold onto his humanity, the cruel and heartless insect takes over and his desperation to stay human overrides his need to protect Veronica, the love of his life and the mother of his unborn child.
 * In Night of the Living Dead (1990), Ben dies and turns into a zombie.
 * Resident Evil (2002). Rain Ocampo turns into a zombie.
 * Also in the sequel.
 * This happens near the beginning of Zombieland. Not someone the main character knew well but still played for drama, and a demonstration of the importance of Rule #2.
 * Darth Vader of Star Wars counts in a physical sense. He became a bad guy before that, but his massive injuries and horrific reconstruction made him a monster that could never be fully whole again.
 * Just about every Universal Horror movie monster ever.
 * Except, and ironically given later portrayals, Dracula. Lugosi's Count was pure Chaotic Evil. Though even he has a wistful moment in the first film when he says: "To die.. to be truly dead... That must be glorious."
 * Shaun of the Dead: When his best friend is turned into a zombie and can't be turned back, Shaun, instead of killing him, locks him up in the storage shed and plays PlayStation with him.
 * The depiction of Two Face in The Dark Knight is particularly heartbreaking.

Literature

 * It doesn't get more tragic than the Creature from Frankenstein. Created and immediately abandoned by its creator for being ugly, before it even really wakes up. Made innocent, his nature is soon twisted by circumstance into something horrible and evil, alone, wretched and violent. The worst part, however, is that he knows exactly how wicked he has become but knows he cannot change.
 * The novel for Coraline has  This gets even worse in the movie, since it's implied that
 * Many in the Deltora Quest series, the most prominent of which was the fate given to Doran the Dragonlover.
 * Diran's best friend from his acolyte days comes back as a werewolf (anathema to members of their church) in the Blade of the Flame books.
 * Erik from The Phantom of the Opera is this, even when he's not portrayed as Draco in Leather Pants
 * Werewolf Remus Lupin in Harry Potter, although he's only like this once a month. The rest of the time he's safe, good, and in his right mind.
 * Even then, it's only if he doesn't have any Wolfsbane potion that the wolf takes over.
 * This occurs in Galaxy in Flames, when Loken meets Kharn after the latter fell to chaos. Kharn had been previously established as a noble and loyal warrior, acting equerry to his Primarch Angron and tempering his rages. Now he's just a snarling, hate-filled... monster - for lack of a better word. When Loken asks what has been done to him, Kharn regains his sanity for a moment and expresses regret, but "there is no going back". Then he goes under again and attacks and Loken has no choice but to try and kill him (of course it doesn't take).
 * Vrasta the "Wrong One" Fammin in Chronicles of the Emerged World has feelings, dreams and a conscience, but when his name is spelled he's compelled to obey orders and.
 * In The Dresden Files book  A bit of a subversion, since

Live Action TV

 * Season 8 of Smallville introduces us to Davis Bloome a likeable paramedic, and possible Love Interest for Clark's best friend, Chloe Sullivan. Unfortunately, Davis has alien Serial Killer and Person of Mass Destruction Doomsday trapped inside of him. Davis shifts into Doomsday more and more frequently as the show progresses, all while trying to repress his Super-Powered Evil Side while committing murders (of criminals) in his human form. Ultimately, it destroys his sanity and he has to be put down for his own good and everyone else's; Davis is killed and Doomsday is separated from him and buried alive.
 * Mitchell from Being Human (UK)

Video Games
""Tommy? I can't feel my legs...""
 * A main plot point of Castlevania Portrait of Ruin..
 * StarCraft:.
 * Not really in that she loves being evil.
 * The original certainly wouldn't have approved of her infested counterpart's actions, so its more of And Then John Was a Zombie than Tragic Monster.
 * And It's more of a Then Let Me Be Evil moment. She was thrown to the Zerg, she's just playing the part because of it
 * While the original game largely avoids this, Wings of Liberty employs it with regards to Raynor's relationship with her. Especially prominent just prior to the campaign's finale when . Then promptly averted when.
 * Cave Story: Toroko, an innocent villager, becomes a raving monster after being force-fed a nasty red flower by Balrog under the wicked Doctor's orders. Before bitching about Balrog, keep in mind who's the thrall and who's the Complete Monster.
 * in Metroid Prime 3: Corruption. Also, if you're not careful, Samus. To be more specific, in the beginning Dark Samus infects all four bounty hunters with Phazon.
 * First, you fight on Bryyo. A bunch of Space Pirates arrive in a cutscene. In a Big Damn Heroes moment,  destroys them all... then attacks you. One boss battle later, the player is treated to a cutscene of  coming to his senses to a moment, then is impaled on a spike of his own ice and absorbed by Dark Samus.
 * Then, on Elysia, you fight . You return to your ship, only to find trying to destroy it.  spots you, and tells you you're just in time.  throws the ship at you, and the boss battle begins soon after., and then Dark Samus absorbs him.
 * Then, on the Space Pirate Homeworld, you fight After the boss battle,  At this point, Samus can't bear to look as Dark Samus absorbs.
 * In Prey,.


 * The Elder Scrolls IV: Shivering Isles: Your old mentor, Sheogorath, turns out to have been an alter ego of.
 * Also, on the way to the Big Bad of the Mage's Guild quests, you are sent to rescue  You find him... a little too late, after he's been.
 * In Final Fantasy IV, Edge's parents are transformed into monsters controlled by one of Rubicante's minions. They eventually recover their minds and commit suicide. Rubicante does express sorrow for this, and apologizes to Edge. The minion did this against the orders of Rubicante's, who implies he would have killed the minion for this were he still alive.
 * The whale-like destroyer Sin in Final Fantasy X is
 * Splatterhouse. Your girlfriend. Stage 5 Boss.
 * In the sequel, it was retconned so that that one was a monster disguised as her.
 * Depends on which version you're referring to. In the Japanese version, your girlfriend is most definitely Killed Off for Real. (Though Splatterhouse 2 essentially serves as a Reset Button for that status, one way or the other.)
 * Annette in Dracula X: Rondo of Blood, if you fail to get her out of her cell beforehand.
 * And again in the remake Castlevania: The Dracula X Chronicles, although the mood is kind of sidetracked since, instead of being turned into a skull sorceress as in the original, Annette is instead transformed into a Stripperiffic succubus in cleavage-barring black lingerie, because Evil Is Sexy.
 * Dracula in many of the games qualifies as such, as he turned into him in the first place because of both loves of his life's deaths.
 * In Crisis Core: Final Fantasy VII,
 * In Alien Syndrome for the Wii, you have to fight
 * In Resident Evil Code: Veronica,.
 * Another one of the bosses, "Nosferatu", is the product of a botched experiment with the Veronica-T virus on Alexander, Alfred's father.
 * William Birkin in Resident Evil 2 is also kind of one of these. Mortally wounded by Umbrella's agents, he injected himself with The Virus out of desperation, turning him into his tragic One-Winged Angel form. Mitigated somewhat by how before this transformed, the human William Birkin was a downright evil man.
 * In Resident Evil 3: Nemesis, you encounter a UBCS officer named Murphy who is turning into a zombie. Either Carlos or Nicolai puts him out of his misery.
 * Lisa Trevor from the remake of Resident Evil. She immediately puts all the above to shame as she was the helpless 14 year old daughter of the mansion's creator, and she and her mother were made into test subjects by Umbrella against their will. Her mother died from being infected with the T-Virus, but Lisa sadly survived and was then proceeded to be injected with the Nemesis virus (which was used to produce the walking nightmare in Resident Evil 3 of the same name) and upon its injection, it created the G-Virus in her body. Making matters worse, even with her mind completely deteriorated and now orphaned, she longed to find her mother in a mansion now completely ridden with monsters...alone....
 * All the zombies/Ganados/Majini from all the games are these. If the viruses and Las Plagas parasites hadn't broken out, they wouldn't have become these... things...
 * In Digital Devil Saga, this happens not once, not twice, but three times.  In the first and third cases, the only cure was to be heavily mortally wounded. In the second case   was too weak to even manage that.
 * In the upgraded version of Baldur's Gate 2: Throne of Bhaal the Big Bad turns your sister Imoen into the mindless Slayer form and sets her on you. She will revert and rejoin your party if you leave her long enough, unfortunately the Big Bad also sets Jon Irenicus, Bodai and a fallen Solar on you at the same time. Taking on all four at once is very difficult, and she's the easiest to kill...
 * This is how Imoen dies in the novelization of the game.
 * This only happens if you use the Ascension mod, not in the bare game.
 * It can seem like this is going to happen in the main Baldur's Gate II storyline, too:.
 * Odin Sphere has two examples: The first is Belial, a dragon that started off rather friendly but eventually becomes a mindless slave. The other is ; though able to return to normal form after the first transformation,   fully transforms just in time for the boss run final book of the game. Oddly enough, to get the best ending you have to.
 * Doom 3 tries to do this with Mission Control Sarge, who gets turned into a cybernetic tank demon about 3/4ths of the way through the game. It fails, since he was always a hardass who constantly railed on your character, and it's unclear as to whether or not most of your interactions with him were after he succumbed to The Virus anyway.
 * It is heavily implied that the sarge nearly immediately succumbed to the virus, and was leading your character (and the rest of the military force) on the entire time. So this isn't really a case of that at all.
 * There's also evidence saying that he was converted by Bertruger sometime after (one theory places the time at when his transmission is cut off while yelling at you for disobeying orders), and that his attempts to call for backup were just Genre Blindness.
 * In the first-person shooter Area 51, one of your teammates succumbs to The Virus about 1/4th of the way through the game, turning into a mutant and forcing you to fight and kill him.
 * And you yourself partially turn into a mutant halfway through the game.
 * The game over screen for the original arcade Area 51 game showed the protagonist morphing into one of the alien mutants, having been infected with The Virus.
 * In Quake IV, Lt. Voss, your squad leader, gets Stroggified and turned into a mech-like boss monster about 1/2 way through the game.
 * The stroggified Kane may be considered one of these, since his cybernetic makeover is permanent, although he still has control over himself, making him a Phlebotinum Rebel.
 * Sergeant Kelly in Doom 3 gets turned into "Sabaoth", a cybernetic demon with a tank for a lower body.
 * In the third party Marathon Expansion Pack, Marathon: RED, at one point the player is captured, infected with The Virus and mutated into a grotesque cyborg-alien hybrid known as the "Reaver", and then sent to kill the surviving humans, but then turns back against his creators.
 * In the first Silent Hill, you have to fight the possessed Cybil on the carousel in the amusement park. You can either kill her, or for a better ending,, to exorcise the parasite possessing her.
 * Cheryl/Alessa becomes a tragic monster if you don't unlock the motorcycle gas tank and hereby allow Kaufmann to obtain the Aglaophotis vial, ie exorcism Phlebotinum. Cue Downer Ending. Luckily not as much a Guide Dang It as the "+ " ending.
 * Most of the monsters in Silent Hill can be interpreted this way. For all we know they're human souls bound to monstruous bodies as punishment. Even Pyramid Head seems to be punished by wearing his signature uncomfortable-looking helmet. It's even easier to see things this way in games where the monsters are easy to beat, as the player character rampages around, shooting any helpless freaks they find.
 * Valkyrie Profile: near the start, Princess Jelanda is transformed into some kind of undead demon and you have to kill her. Then she proceeds to join your party. Later, Lezard transforms  into a monster, and you are forced to kill him. Also, a certain boss in the first game is revealed to be a childhood friend of the main character of the prequel.
 * In Final Fantasy Crystal Chronicles, the player gets the option to
 * Another Crystal Chronicles example, adding to the  it's heavily implied that that the humanoid enemies, such as Skeletons and Sahagins, a fair bit of them are mutated humans, driven insane by miasma and hopelessness. Have fun playing through Tida and Conall Curach now!
 * Have fun? In Tida and Conall Curach?
 * In Onimusha: Dawn of Dreams, you must fight Ohatsu when she has been possessed by the Genma. Tragic music even plays during the battle.
 * Legend of Legaia has Gaza, a mighty swordsman. After being defeated by party, he is taken away by Songi, one of the main antagonists. Later, Gaza shows up again, this time under the control of Sim-seru. He turns into a monster, which the party has to defeat. Gaza is injured fatally, but he makes a noble sacrifice to save his town in his last breath.
 * in Arc the Lad 2.
 * In one of the minor quests in Legend of Mana, one of the professors of the magic university is looking for his brother, who was out exploring a snowfield for faerie treasure. By the time we get there, the professor's brother has turned himself into a snow monster in order to survive, which you have to kill in order to complete the quest.
 * This list would not be complete without  of Persona 3, who turns out to be the Anthromorphized avatar of the spirit of . Fully aware that he's doomed to, he gives your party  , just in time for  . Made even more tragic by the fact that he's actually rather fond of the world now, having spent 10 years  . The battle against him has him spouting off Tarot card lessons in a defeated sounding, near emotionless voice all while bemoaning how "It's useless.".
 * The new Persona 3 Portable throws an even more tragic twist into the situation: if you pick the female version of the Main Character,  becomes an S-Link with romantic potential. So, not only
 * Done to death in Metal Gear 2: Solid Snake, which had the first two of its three climactic boss battles with the protagonist's best friend. Metal Gear Solid called back to it, only now the best friend is a crazy semi-ninja.
 * Done very well in Fire Emblem: Path of Radiance, where King Ashnard of Daein
 * An even better example of this trope from that game would be, though he is not literally a monster. The conversations after the battle push it straight into the territory of an enormous Tear Jerker. Especially Haar's reaction to it, and Jill's conversation with the peasant woman.
 * In Fire Emblem: Blazing Sword (Just Fire Emblem in the Western releases), after Eliwood obtains the dragon-slaying sword Durandal, he is greeted by an enormous ice dragon and is driven by the sword to kill it. Then the Big Bad Nergal shows up to rub his face in the fact that he has just killed
 * Happens In Parasite Eve to the police dog, Sheeva. She goes berserk from Eve's influence and mutates into a 3-headed beast. To make it even more tragic, Ben, her 8 year old friend is forced to kill her.
 * Melissa Pierce herself qualifies, as all she wanted was to be a renowned opera singer and was very excited about getting the role, and was easily manipulated by people she trusted. Makes one detest Klamp and Eve even more.
 * In F.E.A.R. 2,
 * City of Heroes does this in a few arcs. The Terra arc features a strange case in which you must protect an environmental terrorist named Terra
 * The end of the Lady Grey taskforce forces you to fight The last issue of the City of Heroes comic book also featured this story arc.
 * One Malta Ops mission arc has the player meet a woman who is trying to investigate the disappearance of her son Joe, a convict who'd begun to turn his life around in prison. Pursuing the investigation ultimately leads the player to a lab in which Malta Operatives have.
 * In Final Fantasy Tactics, the protagonist's brother Zalbag is teleported to a place of unknown horror by a Zodiac Demon. He reappears a few battles later and is forced by the Big Bad to fight his brother. Zalbag cannot control his body, but is still self-aware, and begs Ramza to kill him.
 * Wiegraf could also qualify as one, although his status as That One Boss may garner a little less sympathy for him...
 * Everyone that's not Ace from Ruby Quest probably counts, as . Thinking about this too hard is not recommended.
 * In The Adventures Of Rad Gravity, the apparent Big Bad is Agathos, a former human who has been mutated into a giant brain. After you defeat him, he returns to human form and reveals that the real big bad is your sidekick Kakos, who manipulated you into reactivating the evil Compuminds that Agathos shut down.
 * In Skies of Arcadia, Drachma spends most of the game chasing after an arcwhale that killed his son (in his introduction, he chastises the main characters because they needed rescue, and doing so caused him to lose track of the whale).
 * Popular in Chris Roberts games. The trope makes its appearance in Wing Commander III, with, and is either played straight or subverted in Wing Commander IV, depending on.
 * Also shows up in Starlancer, with.
 * The number of Chris Roberts games which don't utilize this trope can be counted on one hand.
 * Subverted in Phantasy Star IV where Chaz has to fight an illusion of Alys to get the Megid spell.
 * Knights in The Nightmare has two prominent examples: gets turned into a werewolf and  gets turned into a harpy. You're forced to kill them as well and only death turns them back to normal.
 * The Exsphere monsters ("Exbelua") in Tales of Symphonia, who are regular humans turned into grotesque monsters. Played with a bit as the first time you fight one, the characters don't know that they're fighting a friend until it's mortally wounded, but they get hit hard by the horror once they find out and cannot bring themselves to fight any other attacking Exsphere monsters. Played straighter in Regal's backstory,.
 * We also have in a flashback.
 * However, one of them actually can be saved in a sidequest.
 * from Tales of Vesperia is the epitome of this trope.
 * Oh man, it's like the entire point of Mother 3. That cute little family of friendly dinosaur-dragons you play with in the prologue? . How about your excitable, friendly twin brother?  . Plenty of other animals get this chimerical treatment- look at some of the enemies' faces and tell me they're not screaming. The song's called Tragic Reconstruction for a reason, y'know.
 * From Klonoa there's
 * From Breath of Fire I, we have
 * Definitely the entire point of .hack//G.U.. Let's start from the beginning: Haseo's best friend Shino is comatosed, his newly acquired allies start suffering left right and center. The protagonist also becomes entangled in a Batman Gambit (and is an Unwitting Pawn himself), and finally he gets a shot at killing the true culprit after chasing false ends. This is what the true culprit,  wanted from the beginning, launching a convoluted plan to have Haseo kill him. Why? The Corruption has gotten to him and has resulted in a comatose little sister. He has the required cure that will bring everyone back, but it requires a Heroic Sacrifice, and he knows that I Cannot Self-Terminate. By the time Haseo (and by extension, you, the player) knows about this, you cannot help but feel sorry for the guy.
 * The Undead in World of Warcraft usually come in three varieties: this, Jumped Off The Slippery Slope and Complete Monsters. Of the first variety, notable examples include Thaddius, Sir Zeliek, Madrigosa/Felmyst and the resident Dark Action Girl Sylvanas Windrunner.
 * Don't forget Keristrasza. She knows she's too far gone to be saved, and demands you kill her. "Finish it. FINISH IT! Or I swear by the Dragon Queen you'll never see daylight again!"
 * Penumbra features a certain character becoming infected with the virus, forcing you to kill them.
 * It is also suddenly revealed that
 * from Shadow Hearts: From The New World.
 * Also from that game,
 * Brink from Torchlight. An NPC who joins as an ally near the start of the game, the Big Bad turns him into the first boss via The Corruption, and the player is forced to kill him. Partially subverted as the player character doesn't know Brink for very long at all. However, the NPC ally whom Brink was fighting alongside with at the very start of the game definitely seems distraught after his death.
 * In BioShock (series) 2, in a rather heart-wrenching series of events towards the end,
 * The Big Daddies and Splicers in general are this.
 * Especially the Big Daddies. Watching how gentle they can be with the Little Sisters has been known to give gamers pause in killing them, and the forlorn way they go wandering when no Little Sisters are around is just sad. The fact that they are brainwashed and not acting on their own choices just makes it worse.
 * If you followed the ARG, then
 * Runescape has a subversion with Gregorovic, who was turned into a jester-like monster after making a deal with Sliske for immortality. Normally such a character would be sympathetic, though Gregorovic apparently hadn't thought about whether being immortal had downsides or not. But he had come up with another scheme to obtain immortality before signing the deal with Sliske. He had dissected eeveral elves so that he could learn the secrets of their immortality.
 * This is however played straight with Vanstrom Klause, or as he was known as a human, Ascertes Hallow. Unfortunately, the player character has no choice but to kill him, as he was threatening the Myreque. Though even in his vampyre state, it is shown that he does still have traces of humanity in the Runescape novels, however.
 * His son Safalaan suffers the same fate, but unlike Ascertes, he doesn't need to be killed. It turns out he can be cured - and he is.
 * Appears in Dragon Age - the taint can really mess you up.  And don't get started on Broodmothers. Quite similar to the Dalish Elf's case is Danyla in the Brecilian Forest, a missing member the Dalish clan from the Nature Of The Beast quest whose husband asks the Warden to search for her as a side mission. She's succumbed to the werewolf curse and is still barely there when she's found, begging the Warden to put her out of her misery and take her scarf back to her husband before losing it and attacking.
 * Arguably  himself. There are no reliable sources in the game describing what   were really like (the Chantry is hardly unbiased on the subject), but to reiterate the point above: the taint really messes you up.
 * The final boss of the original Gungrave is
 * In Kingdom Hearts Maleficent leads Riku on to believe that she can help him save Kairi and that Sora has abandoned him. He confronts Sora multiple times over the course of the game and eventually leads to
 * And then there's Roxas' fight with  from Days. After being forced to transform into an Axe Crazy One-Winged Angel form, Roxas wins their duel to the death resulting in
 * Birth By Sleep retroactively does this to, of all people, or, as he was once known,  . What he was known as before he was possessed, stripped of his memories, given memories under the posession, then stripped of the possession and own emotion, leaving a shell trying to become what he once was and failing miserably as it reenacts the behavior of its former parasite out of lack of other models.
 * In a way, Birth By Sleep does this for most of Organization XIII. Lea, Isa, and even the Apprentices of Ansem the Wise seem to be fairly decent people when they have their hearts.
 * Golden Sun The Lost Age does this with the last boss, the 3-headed Doom Dragon, which is made of  They got better at the end thanks to the massive ammount of Psynergy emitted from the Mars Star, so it's all good.
 * Golden Sun: Dark Dawn does this as well, when
 * In System Shock 2 the Many has turned 98% of the Von Braun's crew into its mindless monstrocities. They weren't responsible for what happened, but you still had to kill them anyway.
 * The same could be said for in the first game when you had to deal with SHODAN's minions, whom she infected with her mutant viruses. Edward Diego had you remove her ethical constraints, and how does SHODAN play him back? By making him one of her zombie slaves!
 * BlazBlue: This trope is the reason behind Litchi and Arakune's entire backstory. They were both scientists working under an organization, until an incident occurred in which Arakune lost his body and mind, continuing to exist only as an Eldritch Abomination who eats people. Litchi then set out to find a cure for Arakune, even willing to put him out of his misery herself if she has to.
 * One boss monster in Neverwinter Nights 2 Mask of the Betrayer is one of your party member's long-lost mother. You don't find out until after defeating her, though. The bit while fighting her where she possesses one of your party members also qualifies.
 * A few of the villains, (hell an entire race) qualify as this in the Mass Effect series.
 * Many people think the were also victims of Reaper indoctrination, but it hasn't been confirmed yet.
 * In Mass Effect 2,
 * In the "Overlord" DLC,
 * In Star Fox Assault,, who begs for you to kill him during the entire fight as his body and ship are controlled by the aparoids.
 * In Dark Chronicle, after you defeat Gaspard at the base of Mt. Gundor, he
 * In Final Fantasy Adventure, Amanda gets bitten by Medusa and begins turning into a Medusa herself.
 * In Fable III,
 * Dissidia hints that Sephiroth is one, as Cecil, in a pre-battle quote against Sephiroth, mentions that there's "sorrow in [Sephiroth's] wintery eyes."
 * Mithala in Aquaria was revered as the forefather of the Mithalan civilization. Then the local Path of Inspiration took over and the rites took a darker turn.
 * becomes this in the third and fourth endings of Nie R as a result of  She becomes the True Final Boss in these ending paths. You then have the choice of either   or
 * also turn out to be a lot more sympathetic in New Game+, when you can
 * Vindictus:  Learning of this act is one of the most poignant moments in Vindictus' story..
 * In The Lord of the Rings Online, the Ranger Amdir, who you spend the first parts of the game trying to help as a Hobbit or a Man after he is stabbed with a Nazgûl's morgul-blade during the introductory sequence, becomes one of the Cargûl, lesser wraiths in red who are slaves of the Nazgûl, after succumbing to the effects of the weapon. Book 1, Chapter 1, your very first quest in the Epic storyline after completing the Intro and the Prologue, consists of finding and killing Amdir, who is lost to Darkness at this point.
 * Dark Souls is such a Crapsack World that nearly every enemy in the game is a Tragic Monster. Even the player character is on the verge of becoming one thanks to the Dark Sign.
 * In Xenoblade Chronicles,
 * Asura's Wrath:  Made much more obvious with his battle theme, Bonds.
 * In The Lord of the Rings Online, the Ranger Amdir, who you spend the first parts of the game trying to help as a Hobbit or a Man after he is stabbed with a Nazgûl's morgul-blade during the introductory sequence, becomes one of the Cargûl, lesser wraiths in red who are slaves of the Nazgûl, after succumbing to the effects of the weapon. Book 1, Chapter 1, your very first quest in the Epic storyline after completing the Intro and the Prologue, consists of finding and killing Amdir, who is lost to Darkness at this point.
 * Dark Souls is such a Crapsack World that nearly every enemy in the game is a Tragic Monster. Even the player character is on the verge of becoming one thanks to the Dark Sign.
 * In Xenoblade Chronicles,
 * Asura's Wrath:  Made much more obvious with his battle theme, Bonds.

Visual Novels

 * Fate/stay night. Heaven Feel's . Made much worse after knowing her past and having played the Fate route,.
 * Also and her case is much, much worse.
 * Tsukihime:  Go on, think about his past for a moment, then try telling me I'm wrong.
 * definitely counts as a tragic monster after she's converted into a vampire by Roa/SHIKI.

Web Comics

 * In The Adventures of Dr. McNinja, the Doctor's younger brother Dark Smoke Puncher is possessed by a ghost wizard in order that his friend Gordito and his mother Mitzi will be unwilling to fight back. A good idea in theory...
 * Dracula only has the ghost use the possession spell so that Gordito would have to die (DSP is possessed entirely by accident). There was no intent for the wizard to live through the encounter.
 * Trace Legacy of Twokinds was a pretty nice guy with incredible power. And then he lost the wife he spent so long getting. And then things got worse.

Western Animation

 * Most of the villains in Scooby-Doo! Mystery Incorporated are this. For example, there's Professor Pericles. He was once an innocent parrot who was friends with Mr. E. But unfortunately, the Nibiru Entity corrupted him to the point that he became a sociopath. He also manipulated Professor Pericles into helping him escape from his prison under the pretense that he'll lend him his power. Unfortunately, that wasn't the case, and Professor Pericles dies.