1984 combines this with The Bad Guy Wins, as The Party successfully brainwashes Winston into becoming another one of their drones before killing him.
Breakfast of Champions puts a spin on this. From the get-go, it's a Foregone Conclusion that Dwayne Hoover will become a lunatic who will savagely assault several people. However, despite providing in-verse reasons for his change from a loving, charismatic man to a violently unhinged brute, it's ultimately because the author, Kurt Vonnegut made it so.
This is Geder Palliako's arc across The Dragon's Path, the first volume of The Dagger and the Coin. Initially introduced as a bumbling, nerdy young knight from a minor noble family, the realization that he was set up to take the fall for his country's failed occupation of a captured city state (when taken in tandem with his Fatal Flaws of a Lack of Empathy when it comes to his big plans and a penchant for Disproportionate Retribution when he feels he's being mocked) drives him to his first atrocity, and he spends most of the rest of the book vacillating between saying I Did What I Had to Do and My God, What Have I Done?. Then he meets a creepy cult of spider-worshippers who have decided Geder is their Dark Messiah who will lead their religion in conquering the world, and who have the powers to make him believe it too. By the middle of the second book, Geder is well on his way to full Evil Overlord territory.
The Empirium Trilogy: Rielle's storyline is about her transition from being the long awaited, beloved Sun Queen to the long feared and widely hated Blood Queen, traitor of her kind.
Forest of a Thousand Lanterns: Xifeng starts the book as a good person, but gradually resorts to things like eating her court rivals' hearts to gain power, all for the purpose of eventually becoming Empress.
Gingema's Daughter, the first book in Sergey Sukhinov's Emerald City series, is about the adventures of Corina, originally an ordinary, if somewhat lazy, girl. She starts her way as an understudy of Gingema, then runs away to travel with her wolf companion. She lives by different families, usually helping them magically in secret. But gradually, she decides that Being Good Sucks since everybody bothers you with requests, and being feared is as important as being loved. She deceives the Woodsman to do her bidding by pretending to be the daughter of his former sweetheart and ultimately manipulates him into deposing the Scarecrow, thus becoming the ruler of Emerald city. The rulership she establishes is a Crapsaccharine World: there is food for free and low taxes, but cross Corina in any way and you are dead or turned into a small animal. By the second book, she kills Ellie's parents and becomes a fully-fledged villain.
In Gormenghast, the titular castle, a massive rambling city-state, is also an oppressive social structure where people are locked into their social roles and even their occupations from the moment of birth. No social mobility is possible and nobody has ever seriously tried to challenge the system. That is, until the advent of a kitchen scullion called Steerpike, who tires of being bullied and overworked in the dungeon kitchens. Escaping from the kitchens, Steerpike literally and metaphorically makes his way up in the hierarchy - initially by physically scaling the outside of the Castle. At first, he is a romantic hero who arouses the reader's sympathy. But little by little, his ambition to rise to the very top and supplant the ruling Groan family takes over, with deceptions, manipulation, and finally murder in support of his goal. The boy hero becomes a murdering villain, slowly but surely, across the course of two books.
In Hekla's Children, a fantasy-horror novel by James Brogden, the story starts off as protagonist Nathan Brookes investigation into the discovery of a body in a bog and his quest for redemption in an incident 10 years ago that led to the disappearance of 4 students under his supervision. Much later in the story, he's revealed to be a Decoy Protagonist and through a millennias-old Time Loop is actually the monster that started the whole mess in the first place and then another character is revealed to be the true Chosen One.
In Livy's The History of Rome, which is a record of real events (though entirely based on legend for the earlier parts), 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.
The Horus Heresy has done this for Horus, Fulgrim and Lorgar and Alpharius Omegon.
John Ajvide Lindqvist's Let the Right One In and its Distant SequelLet the Old Dreams Die seems to be one for 12-year old Oskar. Even before he meets Eli, a vampire who is physically and somewhat mentally also 12, Oskar starts out obsessed with serial killers and thinks about hurting or even killing his bullies. After he meets Eli he has no problem when she does kill the bullies. By the time of Let the Old Dreams Die, over 20 years later, Oskar is also a 12-year old vampire alongside his now-girlfriend Eli, and its strongly implied the pair have no issue with hunting innocent families.
The Lightbringer Series does this with Liv, and also provides a fitting quote for this trope itself: "Idealists mature badly; they either become idiots or hypocrites."
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 ExtremistEvil Overlord.
Yarvi starts out The Shattered Sea as a teen Guile Hero who while somewhat more ruthless than your unusual YA protagonist of this type, is still a good guy. After he loses a POV in the second book, he comes off as a more sinister figure and enacts some morally questionable plans, but since they work out for the best, he seems like he still might be the same old Yarvi. However, by the end of the third book, Yarvi is more or less the Big Bad and is a ruthless schemer worse than those he opposes, and is willing to sacrifice his loved ones and everyone else to satisfy his obsession with revenge and self-validation.
The Shining. It starts off with Jack being a happy family man, albeit with a dark past, until the influence of the hotel drives him to madness and monstrosity.
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.
Well of Darkness, first book of The Sovereign Stone Trilogy, provides the origin story for Dagnarus, his lover LadyValura, and his Dragon Shakur (though admittedly, Shakur was pretty evil even before he met Dagnarus). The subsequent two books deal with them as main villains.
Ultimately inverted in The Wheel of Time. After acknowledging that he is, in fact, prophesied Dragon Reborn, Rand alThor is trying to be a fair and benevolent ruler. However, nobles are scheming behind his back, Andor doesnt recognize him as legitimate regent, White Tower kidnaps and tortures him instead of providing support, and so on. So, Rand, battling with stress coming from great responsibilities, increasing paranoia and growing insanity, tries to become harder and to distance himself from loved ones. None of this helps his public image, as even loyal people start to question his authority. Finally he snaps, stops clinging to whats left of his moral integrity and spends a whole book as stone-cold ruthless extremist, inducing primal fear in closest allies and literally spoiling the world around him. And then he is struck by enlightening epiphany, when he is seriously considering destroying the whole world.
Wicked is the Wicked Witch of the West's descent into madness and evil.
The first nineteen or so arcs of Worm describe how Taylor went from a bullied schoolgirl with dreams of being a superhero to Queen of the Brockton Bay underworld. That said, the trope is subverted after that, when Taylor quits the Undersiders to join the Wards, believing, based on Dinah's predictions, that this is the best way to save the world. Double Subverted later, when speaking to another villain who has committed atrocities in the name of saving the world, where she says that she would take it all back if she could, as the price was too high. Taylor's not the only one, either. Alexandria's Interlude shows her progress from an innocent teenager dying of cancer to one of the most powerful superheroes on Earth to an "ends justify the means" tyrant with good publicity.