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.
Subtropes include:
Transhuman Treachery When the original personality is still there, but their allegiance has changed with their form.
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.
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.
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).
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.
Examples (please only put examples here that don't fit into any of the Subtropes):
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Anime and Manga
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 Yusukepretends 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 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 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.
Comicbooks
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).
Films — Animated
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 Were Back A Dinosaurs 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...
Films — Live-Action
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.
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).
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.)
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).
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.
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'stoxin 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.
AoOni, 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.
Webcomics
In The Order of the Stick this has happened to Durkon, who Malack has transformed into a vampire. He currently has no free will (as per D&D rules, which state that newly created vampires are under the control of the vampire who turned them), although even when Malack releases him from his control, (which he apparently intends to do) his alignment will probably have changed.
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.
