The Protagonist's Journey To Villain is a plot in which the protagonist, who starts out well intentioned, turns into a monster. In other words, it's the making of the Villain Protagonist. Sometimes this plot can be backstory, perhaps overlapping with Start of Darkness.
For example: Bob, the happy idealist and doer of good, loses his morals through a series of battles with evil, and becomes just the opposite of what he once was. He is now a cruel, amoral evildoer.
However, note that this descent into evil has to be the focus of the plot, or at least a very important plot point. A mere mention that a bad person was once good is not enough for this trope. This trope is about the journey to evil, not the traveler (Bob), nor the destination.
This is a subtrope of Fallen Hero, in that this is the journey of the Protagonist. Related to Tragic Hero, He Who Fights Monsters, and, of course, the Fallen Hero. Compare and contrast Start of Darkness, where a previously established villain's backstory is revealed. Contrast Redemption Quest.
- Black Lagoon, As the series goes on "outside" character Rock, who is quickly turning into something else. The opposite is true of Revy, who has actually eased up thanks to Rock's optimism.
- Death Note is this for Light. He starts off just killing dangerous criminals, but it's only a few episodes before he convinces himself that since his intentions are noble, and the police are trying to stop him, it's perfectly acceptable to murder them as well. And from there, things go From Bad to Worse.
- This happens to Michi in Osamu Tezuka's original Metropolis manga.
- A good half of the plot of Naruto focuses on the slow decline of Sasuke from angsty, but loyal to full on villainy and the titular character's (thus far unsuccessful) attempts to stop this.
- The slow descent ended the instant Tobi got his claws into Sasuke. With a bit of egging on by the master manipulator, Sasuke dives headlong off the slope; even his teammates who had suffered under Orochimaru were stunned by his sudden swerve into open murder.
- The first half of the third season of Yu-Gi-Oh! GX is this for protagonist Judai. He doesn't remain a villain for long, but comes out the situation an almost completely different character.
- Berserk devotes much of the Golden Age arc to the relationship between Guts and Griffith, and focuses on the factors which would ultimately lead Griffith to betray Guts and become his number one enemy.
- Mirai Nikki, turning The Woobie into... well, something else entirely.
- Mahou Sensei Negima: This may be the fate of poor Negi, who starts the series as a Stepford Smiler brought on by a Dark and Troubled Past. As he and his students become involved in magic and the magical world, he begins to put his students in trouble and he blames himself for everything. In the Magic World arc, things get worse and he learns Black Magic. Now struggling with The Corruption and the danger of becoming a inhuman demon, it's only his True Companions preventing the jump off the slippery slope while his enemies and his master want to push him over the edge. Though whilst his master would rather he were evil, they don't want him mindless. Unlike the enemies.
- Chirin no Suzu has this happen to Chirin. A cute little lamb grows up and turns into a murdering demonic ram.
- The Warrior Cats Expanded Universe manga The Rise of Scourge is about how a cute little kitten named Tiny became Scourge, ruler of BloodClan and Evil Counterpart to The Hero Firestar.
- This trope is the body of the story in Ga-Rei Zero.
- Shakugan no Shana: Sakai Yuuji goes down this road because he's sharing a body with the Snake of the Festival inside Reiji Maigo.
- In the end, this trope is subverted in the final light novel. While incredibly ruthless, Snake of the Festival Yuji ultimately turns out to be the Big Good, permanently saving the day so to speak by ending the Forever War and providing a world for Crimson Denizens to exist without devouring humans' Power of Existence, furthermore allowing the Flame Haze to finally lay aside their weapons.
- The infamous martial arts manga Shamo is about a boy who kills his parents, goes to prison, gets raped, and then carries on to become the most psychopathic martial artist ever conceived.
- This is played with in Kannazuki no Miko. Chikane, being rather attracted to Himeko who already seems interested in someone else, begins to go a tad conflicted. This reaches a head when one of the Orochi Heads uses her desires for Himeko to try and kill her in a scene that remains one of the most well known...for reasons. As a result, Chikane has her way with Himeko, steals Ogami's Orochi mech, kills the other Orochi Heads and awakens Orochi herself. Where the 'played with' part comes up is that Chikane never became evil, she was doing a Batman Gambit to get Himeko to kill her for the world rebirth ritual to be complete and in order to push her far enough to summon a god by herself since a memory of her old self implanted a hatred for said god in the back of her mind. Ultimately, Chikane saved everyone at the cost of her own happiness, but the ending in both the manga and anime suggest that she was given what she wanted in all of her lives but never got.
- Irredeemable is about a Superman Expy called the Plutonian who loses it and starts lashing out. Part of the book involves looking at how he got to that point. And some of his former teammates seem to have started down that same path while trying to stop him...
- Tales of the Jedi: Dark Lords of the Sith is about how Exar Kun and Ulic Qel-Droma turn to The Dark Side.
- In Code Geass: Mao of the Deliverance, Mao's frustration, growing insanity and desperation twist him into a much more manipulative and brutal person. Later chapters, however, appear to hint at a moral recovery.
- This quite amazing fanvid about the Tenth Doctor. The scariest part is that the song (which is also an example of this trope) actually does fit his canon personality to a T.
- The Star Wars prequels are pretty much Anakin's fall from grace. The original trilogy is, of course, his journey towards redemption.
- Harvey Dent / Two-Face in The Dark Knight.
- Page quote, foreshadows his own future and ends up having this as his character theme.
- Fred C. Dobbs in The Treasure of the Sierra Madre. He starts off as our main character, but our allegiance gradually switches to his partners as he comes down with Gold Fever and eventually goes bad.
- The Social Network. It shows Mark's slide from average nerd to a possible Corrupt Corporate Executive due to one mean streak too many. Around the end of the film, he realizes his mistakes, but has somewhat realized he's gone too far to fix them, and tries to make some amends by friend requesting his ex-girlfriend who he insulted over the course of the film.
- Although a lot of the credibility of how villainous he really was is thrown in doubt by the end of the film considering that the film's multiple POVs, none of which are actually Mark's but of the ones testifying against him, are most likely biased. Discussed by Mark and his lawyer in the last scene.
Mark Zuckerberg: I'm not a bad guy.
- Washizu in Throne of Blood. Since the film is based on Macbeth, this is not a surprise.
- Kane in Citizen Kane.
- It was originally assumed that this would be the plot of The Scorpion King, as the prequel to The Mummy Returns, but the film ends on a good note with no indication that Mathias will become evil.
- My Best Friend's Wedding is a rom-com variation of this, although the film is clever enough to hide it under the usual Julia Roberts tropes for the first half of the film.
- The Godfather trilogy is all about Michael Corleone's transformation from White Sheep of a crime family to its ruthless leader, and subsequent doomed attempts to atone. Initially he's not supposed to be involved in the family business at all, as his father genuinely wants someone in the next generation to leave their criminal past behind, but Michael is drawn in in order to protect him from assassination and ends up being the only real candidate to succeed him. He starts out promising his wife that he too intends to make the family legitimate, and his justification for everything is that he's protecting his family. But it turns out he thinks the best way to do that is by consolidating his power and taking out all his enemies in one fell swoop, who happen to include his brother-in-law. The second movie takes the paradox further—now the enemies he's wiping out are a terminally ill man who's no threat to him anyway and, famously, his own brother, and in the meantime his coldness and the violence that surrounds him have driven his wife and children away. The third film has him as a tragic figure realizing that he can't undo what he's done and that the future of the family is out of his hands, and eventually receiving the ultimate poetic punishment: seeing his daughter killed by a bullet meant for him.
- Wicked is the Wicked Witch of the West's descent into madness and evil.
- Although, if you really wanna get into it...
- Mad, yes, but she never gets around to much actual evil (apart from neglecting the kid who is probably hers) because she's busy being insane and broken-hearted and bitter as hell.
- She is implied to (and attempts to) commit murder multiple times over the novel, for increasingly petty and unjustified reasons.
- Feanor's whole arc in The Silmarillion is his descent from hero to Anti-Hero to psychotic, obsessive Villain Protagonist.
- So is Maeglin's, though he of course had the excuse of being caught and tortured by Morgoth.
- In Livy's The History Of Rome, which is a record of real events, embellished where the author felt it necessary, this is a major theme for more than a few of the kings and consuls of early Rome.
- In the Mistborn trilogy by Brandon Sanderson this is the supposed backstory. A thousand years ago a champion, the 'Hero of Ages' rose up to defeat an (unspecified) evil known only as 'The Deepness' but upon his victory he took possession of the world as its Lord Ruler.
"For a thousand years the ash fell and no flowers bloomed. For a thousand years the Skaa slaved in misery and lived in fear. For a thousand years the Lord Ruler, the “Sliver of Infinity,” reigned with absolute power and ultimate terror, divinely invincible."
- The heroes of this story find an old logbook written by the man who would become the Lord Ruler which shows how he began his quest as a humble, earnest man trying to save the world. In the end the truth becomes far more complicated as the Lord Ruler's motivations are slowly revealed throughout the trilogy. The short version is that the hero, Alendi, was duped by prophecies being altered by Ruin, an Omnicidal Maniac deity trapped in the Well of Ascension who would be released if the hero reached the Well and "released" the power. When the scholar who originally prophesized the hero learned the truth, he had his allies pose as guides and murder Alendi when he reached the Well. Then one of the guides named Rashek took the power in the Well and kept it, keeping Ruin trapped and becoming the Lord Ruler. He was driven insane over time by Ruin, becoming a Well-Intentioned Extremist Evil Overlord.
- The Transformers: TransTech story "I, Lowtech" is the first-person perspective story of a Corrupt Corporate Executive trying to figure out why he seems to no longer be in his real body. While he was not exactly good to start off with, he was (technically) law-abiding and never caused direct harm. Until a combo of his first violent act done in self-defense and nobody taking his claims of a body swap seriously makes him realize Evil Feels Good/Evil Is Easy and causes him to start going insane and degenerating into a rampaging serial killer who kills just because it's convenient/for revenge.
- The Horus Heresy has done this for Horus, Fulgrim and Lorgar so far, and seems to be in the process for Alpharius Omegon.
- Clandish "Cybomec" Consto in Stationery Voyagers, not exactly a good guy to begin with, goes completely off the rails after seeing visions of Mezzelwradd. He begins as a burdened-down corrupt cop, and degenerates into a serial killer - before degenerating further into full-blown terrorism.
- The titular Angel has this. A couple of times.
- Breaking Bad is about Walter White's descent from Anti-Villain to Villain Protagonist. Series creator Vince Gilligan stated that his goal with Walter is to turn Mr. Chips into Scarface.
- On Buffy the Vampire Slayer, Willow gets pushed on the journey to villainy in season 6.
- Morgana from Merlin. Woobie Well-Intentioned Extremist to manipulative, chronic smirker.
- Lex Luthor's journey from good to evil in Smallville is the most prominent plot, second only to Clark's journey to Superman, in the first seven seasons.
- One of the best examples of this would be Macbeth. He starts off as a noble person and good guy - a hero returning from war in triumph, but ambition, his wife, and the witches turn him into a monster.
- Sweeney Todd: The Demon Barber of Fleet Street. A barber framed and transported for life for a crime he did not commit by a corrupt judge who wanted his beautiful wife for himself, he returns to London, finds out what happened to his wife and daughter in the meantime though he turns out to have been lied to about the former by Mrs. Lovett, who led him to believe that his wife was dead, and seeks revenge against the judge, leaving a trail of blood and death in his wake that would ultimately lead to him becoming the infamous Demon Barber of Fleet Street.
- The alliance campaign of Warcraft 3 does this with prince Arthas and the Trauma Conga Line that leads to him becoming The Lich King.
- Sarah Kerrigan's plot-arc from StarCraft could basically be described as "heroic moral center" to "Brainwashed and Crazy Dragon" to "Big Bad in her own right".
- Starcraft 2 appears to have retconed this so that the brain-washing was practically another person. It's generally implied that Sarah Kerrigan is not the same person as the Queen of Blades, and that her infestation had left the good Sarah trapped inside her own mind.
- The Metal Gear Solid prequel games - MGS3, Portable Ops and Peace Walker - are this for Naked Snake/Big Boss.
- Heroes of Might and Magic's first Heroes' Chronicles campaign details the rise and fall of Tarnum, a barbarian whose only goal is to free his people from the tyrannical rule of the Bracadan wizards to re-establish the glorious barbarian empire of old. Throughout the campaign, various events cause him to grow more paranoid and ruthless, with the tipping point being his poisoning of all his generals, whom he suspected of treachery. He is eventually ended by King Rion Gryphonheart, the first Erathian king, in a Combat by Champion. The remaining campaigns detail his redemption after he is not admitted to the barbarian afterlife. His final redemption comes in the barbarian campaign of Heroes of Might and Magic IV, where he guides a young barbarian named Waerjak in uniting the scattered tribes in a story mirroring his own, minus this trope.
- Jin Kazama in Tekken 6. Regardless of his reasons for doing it, he plunged the entire world into war and nearly cruelly executed his uncle (it's not like he wasn't enjoying it) based on selfishness and a theory. Jin himself recognizes what his actions have turned him into, even though he's the only one who could have done what he did.
- In Shin Megami Tensei III: Nocturne, the entire game serves as this if you choose the True Demon Ending.
- The protagonist of Zebra Girl slowly goes insane following her transformation into a demon. Her drive to become human again slowly fades away the longer she remains in that form.
- She finally DOES become human again when she betrays her friends, but it comes at the cost of being trapped in an alternate dimension. Whether she comes to terms with her humanity remains to be seen, since she still has doubt after being trapped in that form for so long.
- Pretty much the entire point of Errant Story, as Ian Samael ... changes over the ten-year run of the comic. (Of course, it wasn't entirely his fault.)
- In the Global Guardians PBEM Universe, the general opinion is that this started for Terry "Mister America" Benedict when he testified during the McCarthy hearings as a friendly witness.
- There's also The Dove's slow descent from street-level hero to serial killer, all in the name of fighting crime.
- Dr. Horrible's Sing-Along Blog serves this purpose for the title character.