Face Monster Turn

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 Super-Trope 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.

Subtropes include:

Transhuman Treachery When the original personality is still there, but their allegiance has changed with their form.
Brainwashed/Brainwashed and Crazy/Mind Control When their allegiance has been forcefully changed by subverting their normal personality.
- Being Tortured Makes You Evil/Break the Cutie/Stockholm Syndrome For when the subject has their personality or allegiance changed by more "organic" means.
Tragic Monster When this happens to someone the audience cares about.
Zombie Infectee Where this trope could occur at any moment.
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.
Sheep in Wolf's Clothing When they use their transformation to aid their fellow humans
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).
The Virus When the cause of the transformation is being "infected" by the enemy.
- The Corruption When this gives the infectee evil powers while it mutates him into a monster.
- Reforged Into a Minion When the character is turned into one of the villain's mooks.
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.
- Transformation of the Possessed When demonic possession also involves transforming into a demonic appearance
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.
Split Personality Takeover When a different personality starts calling the shots.
- Super-Powered 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).
Pro-Human Transhuman Despite no longer being human, the character still sympathizes with humans.

Examples of Face Monster Turn include:

Anime

Happens to Death the Kid in Soul Eater while trapped in Eibon's book.
- The whole Dark Side of the series is made of 'insanity' and its effects.
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 Face Heel 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 the Zanpacto 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" (Zanpacto who killed their shinigami before being turned back and went rogue).
In Puella Magi Madoka Magica, this happens to Sayaka; when she doesn't clean her Soul Gem she turns into a witch. This will eventually happen to all Magical Girls in the series.
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.

Comics

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).

Film

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 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 We're Back!, 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...
In the second Resident Evil movie, 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.
The clone troops who were forced to turn against the Jedi by an implanted command in Star Wars.
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.

Live Action TV

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.)
- Half the population of Corinth in Power Rangers RPM were turned into hybrids, thus turning the entire Monster of the Week Scenario into a Xanatos Gambit to the Home Base with them.

Video Games

This is what André really is in Rayman 3: 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).
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.
- Near the end he also transforms your friend Sue and his minion Misery into monsters to help him in the fight (Sue gets better, Misery's fate isn't explored but she was apparently able to send Balrog to help you escape from the Bonus Level of Hell).
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.

Western Animation

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.