Basically this is when one of the good guys becomes a villain not out of choice, but because they were transformed into one one way or another (this makes it a supertrope to several tropes as outlined below). This can also apply to non-sapient entities, such as computers that go nuts, mons that are Turned Against Their Masters because someone else is giving them orders, and so on. As long as it was happy with its allegiance and something other than its own whims changed it, it fits.
When someone with a different allegiance takes control of someone's mind, making them do their bidding.
- Being Tortured Makes You Evil
When brainwashing is accomplished by means of torturing them.
- Brainwashed and Crazy
When the brainwashing is enough to turn the person to violence against their former friends.
- Break the Cutie
When this is accomplished by destroying everything that the person values or holds dear.
- Mind Manipulation
When their allegiance has been forcefully changed by subverting their normal personality.
- More Than Mind Control
When brainwashing together with other factors changes someone's personality or allegiance.
- Stockholm Syndrome
For when the subject has their personality or allegiance changed by more "organic" means.
- Being Tortured Makes You Evil
- Demonic Possession
When something else with a different allegiance hijacks the subject's body.
- Many Spirits Inside of One
When it's done by multiple beings at the same time.
- Possession Burnout
The possessed host's body suffers damage and decay that may quickly destroy it from being possessed.
- Transformation of the Possessed
When demonic possession also involves transforming into a demonic appearance.
- Many Spirits Inside of One
- He Who Fights Monsters
When this figuratively happens to the protagonist into becoming one.
- And Then John Was a Zombie
When this literally happens to the protagonist (assuming their allegiance changes).
- Full-Circle Revolution
When this applies to La Résistance
- Who's Laughing Now?
When this applies to a weak character such as the Butt-Monkey and The Chew Toy who has been picked on endlessly.
- Woobie, Destroyer of Worlds
When this applies to The Woobie
- And Then John Was a Zombie
- Reforged into a Minion
When the character is turned into one of the villain's minions.
- Resist the Beast
When they try their hardest to stop the inevitable monster from emerging and resist the urge to kill, and warn others around them to stay away lest they face their doom.
- Split-Personality Takeover
When a different personality starts calling the shots.
- Superpowered Evil Side
For when the personality is more powerful when it's in control and a little more interested in co-habitation (or at least can't get rid of the "face" personality).
- Superpowered Evil Side
- Tragic Monster
When this happens either to someone the hero knows and/or cares about, or a backstory is given in such a way that their loss of humanity is to be seen as tragic.
- Transhuman Treachery
When the original personality is still there, but their allegiance has changed with their form.
- Cybernetics Eat Your Soul
When this applies to cybernetic enhancements.
- Cybernetics Eat Your Soul
- Unwilling Roboticization
When the person is forcibly turned into a robot or other machine.
- The Virus
When the cause of the transformation is being "infected" by the enemy.
- With Great Power Comes Great Insanity
Being corrupted by an increase in power.
- Psycho Serum
When the serum that gives a person powers also messes him or her up psychologically.
- Psycho Serum
- Zombie Infectee
Where this trope could occur at any moment.
Aversions and Subversions:
- Heroic Willpower
Through sheer willpower, love and loyalty the character maintains their Face status.
- Pro-Human Transhuman
Despite no longer being human, the character still sympathizes with humans.
- Sheep in Wolf's Clothing
When they use their transformation to aid their fellow humans.
- Emergency Transformation
When the transformation is induced to save a character's life. These characters can change alignment, but not all do.
Please only put examples here that don't fit into any of the above subtropes.
- Happens to Death the Kid in Soul Eater while trapped in Eibon's book.
- Urameshi Yusuke pretends this (v.1) has happened when he comes back from the dead the second time. Just to mess with the racist assholes from the Spirit World who'd been sent to vaporize his corpse to make sure he couldn't arise as a superpowered demon. The dub version ends his FaceHeel Turn speech with PSYCH! and in all versions he puts a shoe on his head and sticks his tongue out. Very funny.
- Also worked. Freaked out Koenma very badly too, to a What Have I Done level for a couple of seconds over protecting the corpse. Also any viewers who couldn't keep ahead of the plot and predict the crazy man.
- In Delicious in Dungeon, this happens to Falin. She gets mutated into a harpy-like monster by the Big Bad, and can likely never be human again. She can, however, be close as possible...maybe.
- In the Zanpakuto Tales filler arc of Bleach, the shinigami's Living Weapons were given human forms (turning them into something closer to Equippable Ally) and brainwashed with More Than Mind Control into fighting them. The following arc dealt with clearing up all the "sword beasts" (Zanpakuto who killed their shinigami before being turned back and went rogue).
- In Puella Magi Madoka Magica, this happens to Sayaka and any other Magical Girl whose Soul Gems don't get purified.
- In One Piece, it's implied this happens to Bartholomew Kuma as he turns more and more into a robot, to the point that he says that he will technically die soon since his free will shall be taken away from him by the World Government. Though only a handful of people are aware that Kuma was ever not a monster.
- In Cute High Earth Defense Club LOVE!, the villains are able to turn normal students from the protagonists' school into the Monster of the Week by corrupting them when they're angry about something.
- In Sukasuka this eventually happens to the main character after he starts turning into one of the 17 Beasts. While still able to exert some control over his actions, its a losing battle, and he inevitably becomes the final antagonist of the series.
- Bruce Banner when he turns into The Incredible Hulk (particularly in the original comics where the Hulk started off as a power hungry sociopath who almost considered killing a teenager who knew who he really was).
- In Amulet, this often happens to Stonekeepers who draw too much on their Amulet's power and become overwhelmed by it in the process, or fall prey to the manipulations of the Amulet spirit. Particular mention goes to Emily in Firelight, who turns into a giant, flaming, phoenix.
- Revolutionaries: After he dies, Domitius Major is reanimated by the Talisman... as a nightmarish Worm That Walks zombie that attacks anything the Talisman tells it to.
- In the Magical Girl Crisis Crossover Shattered Skies: The Morning Lights, Sailor Jupiter is hit with an Evil Nut and turns into a Pseudo-Witch, Mami Tomoe and Kyoko Sakura are forcibly transformed into "Witch-Hybrids", halfway between their human and Witch forms, and Cure Fortune is assimilated by an upgraded Droid Jamanen via horrific Orifice Invasion, which converts her into a "drone" Slime Girl under Jamanen's total control.
- Aladdin. When Jafar finally gets his hands on the lamp thus controlling the genie (who has to obey his evil master even though he doesn't want to harm Aladdin).
- In We're Back! A Dinosaur's Story, when the dinosaurs sacrifice themselves to be part of Dr. Screweye's circus to allow the kids to go free. They get scary looking too...
- Vakama in BIONICLE 3: Web of Shadows. While he was shown to be on shaky terms with his team even prior to his mutation into a half-beast, he only turned evil due to Roodaka taking advantage of his bestial impulses.
- In Van Helsing, the titular character gets bitten by a werewolf and turns into one. Although he fights and ends up killing Dracula, he is still a monster and ends up killing his female companion before she can cure him.
- In Resident Evil: Apocalypse, a hero from the first is turned into Nemesis and sent to fight Alice and the heroes. Eventually she does manage to reach him and get him to break their control.
- This happen in almost every zombie film, where even the most heroic character could turn into a villain after being bitten by a zombie.
- The clone troops who were forced to turn against the Jedi by an implanted command in Star Wars: Revenge of the Sith.
- In the Street Fighter movie, this is the goal of Bison's Super Soldier program, starting by turning Guile's friend Blanka into a green monster. Thankfully, while he was being brainwashed the scientist in charge switched the "Evil" off and turned the "Good" on, allowing for a 40 / 60 % mix of Evil to Good, respectively (more or less, it's been a while).
- In Indiana Jones and the Temple of Doom, Indy is made into a zombie-slave, though later recovers.
- Star Trek: The Original Series: in the episode "Catspaw", some of the crew are made into zombie-slaves by the witch-alien.
- Previously, it had happened in "Return of the Archons", where Sulu is turned into one of Landru's followers.
- Jean-Luc Picard being turned into Locutus of Borg, in Star Trek: The Next Generation.
- Star Trek: Voyager: in the episode "Equinox", when the captain of the titular ship deletes the Doctor's ethical subroutines, and then has him essentially torture Seven of Nine.
- Subverted, inverted and played straight in the case of Angel from Buffy the Vampire Slayer: he was a jackass who was turned into a monster, was forcibly given back his soul and instead of returning to his jackass original personality, became a good guy, who was then turned back into a monster, and back into a good guy several times.
- It's the reason he doesn't go by his former human name even when he's a good guy, he's NOT that person anymore. By the time he gets cursed with a soul, he'd been a vampire for about five times longer than he was ever human. He still considers himself a monster, just one capable of feeling guilt for his actions.
- In Kamen Rider Kiva, a villain turns Wataru into a scary-looking CGI monster and brings him under his control. He eventually breaks free and uses the monster form to kick his would-be master's ass. We see "Kiva Emperor Flight Style" only once more after that, sadly.
- In Power Rangers, this happens to civilians on an "a few times in some seasons" basis. A person is turned into a Monster of the Week, and acts like a Monster of the Week until he is freed. Whether blowing up the monster form with a Finishing Move will free the person or is something that would kill him so the Rangers have to come up with something else varies (though in Power Rangers Dino Thunder, kaboom always works.)
- Dean is transformed into a demon by the Mark of Cain in the S9 finale, and remained so for four episodes before being cured.
- Sam underwent something similar in S6 due to Castiel accidentally resurrecting him without a soul.
- In Charmed, it became a Running Gag how often each of the girls got transformed into a wendigo/vampire/demon/Fury and had to be saved by the others. (They also turned into good creatures a lot, too.)
- Happens often in Greek mythology, as looking like a monster is apparently synonymous with being evil. Scylla and Medusa are both well known examples: women who were cursed for things that weren't their fault and became vicious and destructive as a result.
- Tamamo-no-Mae Asahi no Tamoto: Lady Hatsuhana a.k.a. the titular Tamamo-no-Mae was a decent woman chosen to become the Emperor's concubine before she is possessed by an evil fox spirit for no apparent reason. She then sides with Prince Usugumo to start a coup against the Emperor and take over the country.
- The penultimate boss in Freedom Planet is Milla, who has been corrupted by Lord Brevon into a monster. Lilac or Carol is forced to kill this character in order to advance. Or so it seems. Milla survives, but just barely. However, playing as the spoiler character skips this boss battle entirely, since the character can't fight themselves.
- This is what André really is in Rayman 3: Hoodlum Havoc: a red lum who got scared, turning it into an evil black lum.
- In MOTHER 3, Lucas's twin brother Claus (who was either seriously injured or dead) is turned into a cyborg and mindless slave by the Pigmask Army.
- But we knew he was in there.
- In Resident Evil Outbreak File #2 the third series Tyrant helps the players at first (since it's been programmed to protect humans and destroy hunters). Then it gets damaged, loses its Power Limiter, mutates even more and goes crazy.
- Also, if a player dies then they turn into a zombie (or a more powerful Roaming Enemy in one level).
- In Resident Evil Code: Veronica, Steve is infected with T-Veronica and attacks Claire until The Power of Love returns him to normal (but he dies immediately after).
- Even as far back as the second game, Brad Vickers survives the first Resident Evil just to show up in 2 as an optional zombie encounter. 3 shows how he was zombified.
- In Cave Story, The Doctor feeds red flowers to one of your allies, turning her into a frenzied monster and forcing you to kill her.
- In Quake IV, the player character is Stroggified, but his comrades rescue him before his mind control chip can be activated, thus making him a Phlebotinum Rebel.
- In Zenonia 3: The Midgard Story, Ekinard's toxin leaks into the rivers on Maru Island and causes anyone who drinks the water to turn into a monster.
- Every single enemy bar the final boss in Wario Land 3 turns out to be a human turned into a monster by the Big Bad as revenge for sealing him away.
- Ao Oni, specifically, version 6. Takeshi even tries to kill himself to avoid this. It doesn't work.
- Already teetering on the edge of being utterly mental, Tragic Hero Siegfried Schtauffen in the original Soul Edge is completely consumed by the eponymous Evil Sword when he finally takes possession of it, turning him into the Soul Edge manifest; a rampaging demon-knight known as Nightmare.
- In Disney Princess Enchanted Journey, the Bogs are actually captured butterflies that were forced into evil, and return to their natural state once defeated.
- While not on the "Good" side, the scientists turned into purple spider-zombies in Azure Striker Gunvolt possibly count
- In Shadow Hearts: From the New World, Edna Capone suffers from this after being revived with the power of Malice.
- Blackarachnia from Transformers Animated actually turned to the side of the Decepticons after mutating into an organic spider.
- Wasp also became a Decepticon after Blackarachnia mutated him into an organic wasp shortly after he vowed revenge on Bumblebee for (accidentally) sending him to prison, though he'd been trying to take Bee down for some time before. The Decepticon symbol his new form inexplicably has is basically there 'cause the toy has it.
- In Miraculous Ladybug, the main villain, Hawk Moth, has the power to turn normal people into the Villain of the Week by preying on them in moments of anger or depression. One of Ladybug's powers is to purify the akumas that possess them, turning them back to normal.
- Happened in the backstory of Steven Universe. Those monsters the Crystal Gems are always fighting? Theyre other Gems. At the end of the Gem War, Homeworld deployed a Fantastic Nuke that mutated and corrupted every Gem caught in its range. Pearl, Garnet, and Rose were the only Crystal Gems who escaped it and they spent most of their time since capturing their former friends and enemies to stop them from hurting people.