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, endures a Trauma Conga Line, loses his loved ones and his morals through a series of battles with evil, and becomes just the opposite of what he once was. He is now a cruel, immoral 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 the primary arc in many a Tragedy, which usually ends with the death of the hero-turned-villain as its source of audience catharsis.
This is a subtrope of Fallen Hero, in that this is the journey of the Protagonist. Related to Tragic Hero, He Who Fights Monsters, The Paragon Always Rebels, Face–Heel Turn, Became Their Own Antithesis, and Used to Be a Sweet Kid. Compare and contrast Start of Darkness, where a previously established villain's backstory is revealed. Compare Big Bad Slippage, where a character who may or may not be the protagonist becomes the Big Bad over the course of the story, or Sudden Sequel Heel Syndrome, where the "journey" happened off-screen between installments and at best might be elaborated upon. Contrast Redemption Quest and Rogue Protagonist, where the main character from a work becomes the villain in the sequel.
SPOILER WARNING! In many cases, the mere fact that this trope applies to a work can be a spoiler. Read at your own risk.
- In the Yu-Gi-Oh! card game this happens to Gagagigo who started his career as the little and cute Gigobyte and fights along with Eria the Water Charmer. As the grown-up Gagagigo he eventually left her and someday he fought Freed the Brave Wanderer, ended up being trapped in another dimension. He met Marauding Captain and fought against Inpachi, who later appeared again as Blazing Inpachi; the Marauding Captain took the bullet, which inspired Gagagigo to do the same for one of the Captain's men during the war against Invader of Darkness. Later, hoping to defeat Invader of Darkness, he asked the Mad Scientist Kozaky to make him stronger, and Kozaky rebuilt his body and turned him into the corrupted Giga Gagagigo. When he fought against Freed the Brave Wanderer again in his native dimension, he got his own attack reflected and lost. Obsessed with gaining strength to defeat his rivals, he continued his rampage and eventually transformed into Gogiga Gagagigo and truly lost his soul.
- The following story (which is "written" many years after his transformation) inverts his dark development as he fought Freed the Brave Wanderer again, finally overpowering him, but Marauding Captain appeared and protected him before Gogiga Gagagigo delivered the finishing blow. Instead of following his corrupted instincts, Gogiga Gagagigo understands Marauding Captain's actions and forsakes his quest for power. Thus, he finally becomes the strong warrior of justice he once sought out to be, Gagagigo the Risen.
- In The Bible, David goes down this path. Despite being the runt of his father's litter, David becomes God's chosen one and he replaces Saul as Israel's king. For a time, David brings prosperity to Israel, and he is renowned as a hero by basically everyone in his country. David eventually lets the power get to his head, and he indulges in every kind of pleasure he can think of. One day David spots a woman named Bathsheba taking a bath, who was the wife of an officer in his military, and he falls madly in love with her beauty. Knowing that he can't steal Uriah's wife, David conspires to put him in a risky battle, hoping that he will be killed so he can take Bathsheba for himself. David goes through a long period of guilt over the remainder of his life, suffering the death of a child with Bathsheba, losing favor with some of his royal court, and finally ending a rebellion hosted by his son Absalom who died in battle. David believes very strongly that these misfortunes were God's punishment for his sin, and he spends his old age seeking forgiveness from God. Even though David fell from grace, the Bible notes that Solomon — the son of David and Bathsheba — brings even greater prosperity to Israel than even his father did.
- In Norse Mythology, there are quite a few myths starring Loki as a Guile Hero for the Aesir. Then he orchestrates the death of Balder, his motives for which are open for interpretation, and confesses to the crime while giving every other god "The Reason You Suck" Speech at a party. After that he gets rather painfully imprisoned and is destined to lead the army of Helheim during Ragnarok.
- Warhammer 40,000: The Horus Heresy series discusses the events that ultimately lead to Horus betraying the Emperor, such as his brush with death where the Chaos Gods appealed to his repressed ambitions.
- Warhammer: Age of Sigmar: The novel "The Godeater's Son" explores this trope, as it follows the protagonist Heldenarr Fall from a peasant living in Aqshy's wastelands to a powerful Chaos Lord as he seeks revenge against the regime that's been oppressing his people.
- Ebenezer explores how Scrooge became the cold-hearted miser he is at the start of A Christmas Carol.
- Hamilton follows Alexander Hamilton as he goes from an idealistic, eager revolutionary to a bitter, pragmatic politician who is forced to play the game and throw others under the bus in order to protect his reputation and gets what he wants. He eventually has a Heel Realization following the death of his son, who was killed trying to defend his father's honor in a duel, and is a more sympathetic character for the rest of the show.
- One of the best examples of this would be Macbeth. He starts off as a noble person and a good guy – a hero returning from war in triumph. But ambition which was fueled by his wife and the witches leads him to murder his king and usurp his throne and turns him into a monster.
- Othello starts out as a noble and good, if a bit daft, leader, but Iago takes advantage of his jealousy and manages to get him to murder his own wife in cold blood.
- 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 because she wanted him for herself), and seeks revenge against the judge, leaving a trail of blood and death of innocents in his wake that would ultimately lead to him becoming the infamous Demon Barber of Fleet Street.
- Euphoria: While our protagonist Keisuke Takato already starts off rather morally ambiguous with his extremely unhealthy sexual deviancy, he at the very least has enough of a conscience to try and keep said deviancy under wraps until the Deadly Game forces his hand. Going through with the "Brute End", however, quickly has him lose all sense of shame to fully indulge in his sadistic desires, to the point that by the end of it he brutally murders one of the girls and condemns the others to a Fate Worse than Death.
- Raging Loop: Protagonist Haruaki Fusaishi is forced to repeatedly participate in a week-long game of werewolf in order to get out of "Groundhog Day" Loop, and gets increasingly darker as the game loops and repeats. The first time around, he's only a bystander who cannot directly get involved in the game, therefore making him responsible neither for killing villagers nor lyncing suspected wolves. In the second game, Haruaki is a villager and is forced to take several morally objectionable acts to flush out the wolves, which includes sending innocent people to the gallows to keep his Obfuscating Stupidity. In the third game, he's forced to become a wolf, and personally kills several of the people he's come to know over the course of the last two loops to finally end the loop. 'Luckily' the latter loop leads to the victory of the real villain behind the loops, forcing Haruaki to loop back for a final attempt and Set Right What Once Went Wrong.
- Pretty much the entire point of Errant Story, as Ian Samael ... changes over the ten-year run of the comic.
- The other person on the receiving end of the same power-up actually went the other way, from a fairly antisocial and useless character to an active force for good. So the story is at least heavily implying that it was, in fact, Ian's own inability to deal with his issues that screwed everyone.
- In El Goonish Shive, based on what Pandora tells him, Tedd thinks that without his friends he might have taken the same path as Lord Tedd, one of his Alternate Universe counterparts who turned evil.
- Schtein's arc in String Theory (2009).
- 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 banished to an alternate dimension. At the resolution of the plot arc, she's seen to embrace her semi-former humanity AND her demonic essence, regaining her demonic form and abilities while rekindling her human compassion.
- Played for Laughs in Allison Pregler's reviews of Charmed (1998), due to the protagonists' Designated Heroism and her own Alternate Character Interpretations.
"I just got it. This show was actually playing us the whole time! The villains were the heroes, and the heroes were the villains! This was all leading to an Anakin Skywalker moment! We're presented with protagonists with massive power, destined for greatness, and the potential for immense good. However, slowly but surely, we're watching their downfall, leading up to the creation of three Darth Vaders. Out of the most unlikely circumstances, their own protege and her sister, raised by evil, must rise up and save the world. I should have seen it coming. Goodness gracious, Charmed, you Magnificent Bastard! You played me for a fool, but you were pulling the strings all along!
"Or maybe this show is just fucking moronic, you choose."
- Both protagonists of Arby 'n' the Chief make this journey in Season 7. After spending most of the season isolated in real life from everyone except each other, having to grapple with their own mortality and crumbling bodies, and being repeatedly subjected to the douchebaggery of other players on Halo: Reach, Chief and eventually Arbiter both fall under Eugene's sway and join him in his Fragban spree across the network.
- Dr. Horrible's Sing-Along Blog serves this purpose for the title character. Although it's more of an Ineffectual Sympathetic Villain Protagonist's Journey To Not So Harmless Tragic Villain.
- Soviet Womble's Random Arma 3 Bullshittery Part 9 is this, telling the story of a resistance groupnote fighting against the Russians, and slowly becoming more and more evil. Especially noticeable with Soviet, who acts as the most consistent Token Good Teammate throughout the whole thing, believing that they should be fighting for freedom and democracy, and is horrified by the some of the stuff his fellow players do, but by the end he's so worn down that he joins in using civilians as human shields during an attack on the Russian base.
- The core of Arcane is the story of how a fairly innocent child named Powder turned into the psychotic anarchist Jinx.
- DuckTales (2017): Enforced and overseen by Black Heron. Bradford Buzzard had a noble goal in mind when he presented his plan to Ludwig von Drake as a young man, even though he failed to recognize the villainous nature of his suggested method. From the moment Heron allied with him, she began strong-arming him into making concessions towards evil, with adding the F to F.O.W.L. being the first example. Bradford was ultimately responsible for his own choices, but Heron was dead-set on making a villain out of him — and succeeded beyond her wildest expectations, to the point even she feared his wrath during their final years together.
- Infinity Train Cult of the Conductor, follows Cracked Reflection's minor antagonists Grace and Simon of the Apex, introduced as passengers who rampage through the train and terrorize the inhabitants. As they find themselves separated from their followers and travel across the train to find them, they learn certain truths that begin to affect their worldview. Simon starts off as the more sympathetic of the two due to his abandonment issues, only for him to get worse mostly thanks to Grace's Toxic Friend Influence enabling his hatred for the train's denizens to the point where he projects his anger for his former companion at the accompanying Tuba by murdering her. And when confronted with the truth about their cult's conductor, Simon is unable to accept it even when Grace outright admits her lies to him. In response, he doubles down on the cult's beliefs and changes it for the worse, at the cost of his sanity and tragically ends up Dying Alone.
- Star Wars: The Clone Wars served as an expansion on the circumstances that led to Anakin Skywalker becoming Darth Vader, with his turn to The Dark Side explored in a much fuller degree and allowing for his Character Development to properly blossom. Its final scene is of Vader presumably reflecting on it.
- In episode 4 of What If…? (2021), it presents an Alternate Universe where Doctor Stephen Strange lost his girlfriend Christine Palmer instead of the use of his hands in the fateful car crash that would lead to him becoming Doctor Strange. In his grief, he has spent centuries absorbing mystic beings to gain enough power to reverse the Absolute Point regarding Christie's death. In the end, he kills his Good Counterpart and succeeds in overcoming the absolute point, only for Christine to end up not being alive again for more than a few moments... Because Christine Palmer's death in this timeline was supposed to kick off the events that lead to Doctor Strange becoming the sorcerer he is now, Strange Supreme's success in undoing the Absolute Point regarding her death ends up destroying the universe he once vowed to protect.
- The short-lived cartoon Whatever Happened to... Robot Jones? would have been one of these if it hadn't been canceled so early. It was supposed to end with the titular protagonist leading a robot rebellion to wipe out humanity.