Most literary heroes are fairly simple, if heroic, characters: They start from humble beginnings, they're called to adventure, they defeat the bad guy, and then they die and/or get married. Order is restored. The book ends. That's about as basic as it gets... for most stories, anyway.
Sometimes there are a few twists and turns along the way, like when our hero doesn't defeat the bad guy. Or when our hero is the bad guy. The trope of literary villains redeeming themselves is fairly well established, but sometimes it's actually our heroes who turn to the dark side (or who turn out to have been secretly working for the dark side all along). Here are a few literary heroes who turned out the be the villain.
The space between "hero" and "villain" can get surprisingly messy. For one, antiheroes are very popular as protagonists—think grizzled tough guys or characters with complicated morals, like Hamlet or Catwoman. But this list isn't about them. This list is about people who seem fine and dandy (perhaps with a few heroic flaws) on page one, and turn out to be screamingly evil by the book's climax. We thought the story was going to be about these characters changing for the better... but we thought wrong. Very wrong:
Say it with me, now: Frankenstein is the doctor, not the monster. Except... Frankenstein kind of is the monster. At the start of the book, Victor Frankenstein is a young scientist grieving for the recent death of his mother. He's brilliant and determined, and he creates a living creature out of dead human tissue... and then abandons it. Yeah, the monster isn't really the villain of the original novel. The monster is essentially a baby, who grows up to be thoughtful and compassionate, while Frankenstein continues to hate and fear his "offspring." Victor isn't a scheming villain, but he is a cruel and callous deadbeat dad (or deadbeat creator, at least).
The world of comics is full of characters who ping pong from good to evil and back again. Jean Grey's transformation into Dark Phoenix is one of the most notable, though. Starting with The X-Men #1, she's the team's kindest, most nurturing character. She's also the token girl. But Jean manages to subvert her girly, damsel-in-distress vibes by transforming into an immensely powerful villain: Dark Phoenix, who consumes a star system and jeopardizes the entire universe.
Yeah, yeah, we all know that Jack from The Shining is the villain. But that's only because the book and film adaptation have become such huge cultural touchstones. At the start of the book he's just a writer and recovering alcoholic who wants to repair things with his family. He's flawed, but he's the hero. He's actively trying to become a better man, and tells his wife to leave him if he ever starts drinking again... but then he gets infested with ghosts and just starts trying to murder everybody all the time. At least you tried, Jack.
If three witches stop you on the side of the road and tell you that you're going to get a huge promotion, your next logical step is to murder your boss... right? Macbeth starts off his play as a classic hero: He's literally on his way home from winning a war. But after hearing a prophecy that he's going to be king someday, Macbeth and his wife start murdering all their friends, including the king himself. The witches don't tell Macbeth he needs to murder everyone to make the prophecy come true, but he slides into villainy pretty freaking quickly as soon as he thinks he's destined for great things.
Poor Catelyn. All of the surviving Starks are forced to do things they might have once considered morally wrong, but only Catelyn gets to come back to life as a revenge zombie. At the start of A Song of Ice and Fire, Catelyn is trying to seek justice for her injured son. She loves her children and wants the guilty parties to pay. But by the end of the series, she's dead, she thinks all her children are dead, and she's been resurrected as the vicious Lady Stoneheart, who murders basically anyone who crosses her path.
Heathcliff goes from poor orphan boy to romantic hero to creepy, creepy dad. In the first half of Wuthering Heights he's a tortured hero, sure, but he's still a hero. He struggles with his all-consuming love of Cathy, and finally leaves, Gatsby-style, to make money so he one day win her back. But then Cathy dies and Heathcliff dedicates the rest of his life to making everyone miserable, abusing his family, and forcing his kids and Cathy's kids to hook up.
Amy from Gone Girl doesn't exactly go from a chill, laid back wife to a criminal mastermind—but she sure makes it look that way. At the start of the book, she just looks like an innocent victim. It's only once we learn that she faked her own death that the fun, villainous Amy comes out. And what makes Amy such a fun pro/antagonist is how good she is at hiding her own evil streak—and how she uses her twisted evil genius to undermine the patriarchy (kind of).
Dorian goes from "pretty boy who should maybe be nicer to people" to "murderer" pretty quickly. At first his crimes are mostly selfishness and being insensitive, but as he realizes that his actions have consequences, he starts killing people to cover up his tracks. Even when he tries to be a better person, he's still doing it to avoid his own personal damnation. Turns out that when you have a painting that ages for you and absorbs all of your evil deeds, you don't have a lot of incentive to be a good guy.
Everyone in the graphic novel Watchmen is kind of an antihero—that's kind of the point. But Adrian Veidt, or Ozymandias, is the only one who goes from all-around good guy to blowing up Manhattan. He starts off as a benevolent billionaire who's trying to bring a serial killer to justice. But then he stages a "alien invasion" that levels New York City, killing millions. At least he's smart enough to wait until after he's succeeded before giving his villain monologue?
From angel to ruler of hell is the ultimate "hero-to-villain" move. Satan is a hero (or antihero, if you prefer) of Milton's Paradise Lost. But the tragedy of Satan is that, in trying to escape his subservient role in heaven, he winds up being trapped in the role of ultimate villain. He might claim it's better to reign in hell than to serve in heaven, but he also gets turned into a snake, which isn't that great either.