Evil Makes You Monstrous

Characters who decide to toboggan down The Dark Slide will have a fun time of it... at first. It starts small, maybe an Evil Makeover with Spikes of Villainy and Red Right Hand. But there's a nasty surprise at the bottom of this insidious slide; those who commit especially heinous crimes, abandon all of their humanity for power, or allow their inner darkness to bloom into a blight won't just grow tiny fangs or discover Evil Makes You Ugly, they'll find out Evil Makes You Monstrous.

This is usually a feature of legends, both in real life and in stories. Basically, a guy, gal or even animal becomes so mean, malicious, and generally Evil that they transcend their mortal existence and become a monster of legend. A scourge on man and beast. After killing enough, the Serial Killer goes from a man into an force of nature that just. Won't. Die. The witch buried in the heart of the woods will rise from her grave and haunt it as a half-dead corpse until her spellbook is destroyed. The man who killed and ate his family out of hunger transforms into a Wendigo. The Creepy Child everyone picks on lets the hatred fester and ferment until she becomes a living poltergeist.

Then again, the (mostly) innocent might suffer this fate because of a Curse or The Virus. In the most extreme cases the character will no longer be flesh and bone but Made of Evil. While it's unlikely the character wants to seek redemption, it's not impossible that a loved one from before the change can at least give them pause.

Can also caused through The Power of Hate. See also/compare One-Winged Angel, Evil Makes You Ugly, and Scaled Up. Promethean Punishment is related, but the transformation into a monster is forced by an outside faction.

Anime and Manga

 * Humans in Soul Eater who devour other humans' souls become Kishin Eggs, eventually becoming a Kishin, superhuman monstrosities who cause insanity in those around them just by existing.
 * Wolf Guy Wolfen Crest: When  is taken over by his bloodlust and obsession with killing Haguro, he becomes a deformed ogre instead of a werewolf.
 * Berserk has the Apostles, every one of whom was once human before making their sacrifice to the Godhand and being reborn as demons.
 * in Puella Magi Madoka Magica embody this trope.

Comic Books

 * Wendigo, particularly the Marvel Comics version. (Only 98% though - the rabid man turns into Wendigo more or less permanantly, not only on the anniversary.)
 * In one issue of the Valiant Zelda comics Link steals the Triforce of Power from Ganon. As a result he starts turning evil, and nearly completely transforms into a piglike monster similar to Ganon.
 * In The League of Extraordinary Gentlemen, Dr. Jekyll indicates that Mr. Hyde has grown much larger from the exercise of sin.
 * Marvel villain The Hood has a red hooded cape that gives him magic powers—and is warping him into a demon.

Film

 * This is the nature of Jason Voorhees in the Friday the 13 th series. Notably, he started out as an implacable but very human killer, until he morphed into an immortal undead killing machine over time.
 * And on a vaguely related subject, Freddy Krueger of Nightmare On Elm Street somehow managed to become an undead dream-dwelling Humanoid Abomination just by being really nasty to kids.
 * Halloween's Michael Myers went from a super-strong, sociopathic human with plans to kill his sister to a completely unkillable supernatural being hell-bent on massacring half of Haddonfield. And if you follow the sixth movie's canon, his power is making him grow bigger in each movie.

Folklore

 * This is one of the possible origins of a vampire, according to some accounts.
 * And werewolves of the cannibalistic Serial Killer type.
 * This is also common in Native American Mythology. Many, many monsters are spawned due to someone acting in such an anti-social fashion for so long that they become monsters.
 * In the Native American mythology you may turn into a Wendigo if you become a cannibal.

Literature

 * This happened to Lord Voldemort from Harry Potter. He lost his handsome looks and acquired ugly snake-like features after using too much Dark Magic. He also made himself very, very difficult to kill through the same means.

Live Action TV

 * In Supernatural Wendigo are created when humans dine on human flesh. The hunger grows and grows until they're nothing but beasts.

Tabletop Games

 * Fomori in Werewolf: The Apocalypse tend to be created when the Wyrm warps the flesh of those who are corrupt.
 * Slashers in the New World of Darkness are people who have done such evil things that the evil has warped them and given them special abilities to help them kill.
 * Similarly, the Centimani from Promethean: The Created. Usually driven over the Despair Event Horizon by the raft of crap Prometheans have to deal with, they turn their back on the Pilgrimage and start dealing with Flux. This usually results in mutations. Lots of mutations.
 * That would explain the name.
 * In Warhammer Fantasy Battle and Warhammer 40,000, followers of Chaos will often mutate, sometimes rather quickly. Chaos mutations tend to be rather...grotesque, or even horrifying. Eventually, this will be how the apotheosis into a Daemon Prince would eventually work, although succumbing to the mutations and becoming a Chaos Spawn is a much more common fate.
 * In Dungeons & Dragons, a lot of The Undead work this way. Cannibals rise from the grave as ghouls, unrepentant murderers come back as wights, those consumed by hatred become wraiths or spectres, and so on. Liches are Immortality Seekers who do it to themselves on purpose.
 * In Ravenloft, this is a game mechanic when making Dark Powers Checks. There are thirteen steps from Purity to True Damnation, and each failed check takes a player one step closer. (Unless, that is, a player commits an Act of Ultimate Darkness, which often causes them to skip one or more steps.) The seventh step (called "the Beast, where the character is often - but not always - beyond redemption) causes a slight physical change, but not true transformation, step eight (called "the creature") enhances the change, giving the cursed character a few dark powers and weaknesses, and the ninth ("the monster", where the character is almost always truly damned) is where the character truly becomes an inhuman monster. For example, take a woman forced to become a harlot who eventually embraces her role and becomes a seductive vamp. When she reaches Stage Seven, she might have Cute Little Fangs and fiery red hair. At Stage Eight (where she has likely used her skills to Honey Trap victims and kill them) she might find herself physically stronger and start to see spiders - which seem overly fond of her - as adorable pets. At Stage Nine (where she has become so depraved that she intentionally looks for pure and innocent victims to corrupt) she becomes an actual red widow, a foul temptress who seduces men not just to kill and rob them, but feed on them as well, having a monstrous body that matches her dark soul. While she may welcome this change, there is truly no going back, and the Dark Powers will ensure  she never escapes their grasp...

Video Games

 * In Kingdom Hearts I, Maleficent embraces the Darkness and becomes a giant dragon as a result.
 * As Sorceresses in Final Fantasy VIII grow older and continue to use their magic, their power begins to change their appearance into something less human and more monstrous. Edea was a fairly young sorceress so her appearance (while slightly altered) was still beautiful, whereas the older (and far crueler) sorceress Adel had morphed into a monstrous, muscular being. However, Sorceress Ultimecia's appearance is up for debate - her true form might be that of a sexy, winged scantily-clad woman, or her true form might actually be her barely-human existence-sucking final form. Considering how the other sorceresses look, the latter is more likely.
 * Then there are the various sorceresses the player must fight during Time Compression. The first batch have odd skin colors and facial markings but still resemble beautiful women, the second batch look a little less human and have eerily warped laughs, while the final sorceress resembles a fat, bloated, purple slug.
 * In Final Fantasy X, this is how.
 * The Legend of Zelda's Ganondorf, tapping into the full power of his Triforce of Power, transformed into the pig beast now known as Ganon. Although he is sometimes able to revert back to his human form at will, he definitely received the absolute refusal to actually stay down.
 * In Diablo II, the protagonist from the first opus, after defeating the eponymous Demon Lord, has put his soulgem on his own body in an attempt to contain his evil. As the game progress, Diablo takes over his body, gradually making him more monstrous until he turns back into the hideous Demon form Diablo used to assume.
 * The further you go down the Dark Side in Knights of the Old Republic, the uglier you get. When you're maxed out on Dark points, your eyes turn red and your skin gets ashen and veiny.
 * In the Resident Evil series, most enemies are formerly-human, and all are monstrous. While there's not shortage of BOWs who were transformed against their will, there's also quite a few (mostly named villains) who did it to themselves on purpose, and they tend to be much more monstrous than the rank-and-file mooks.