In the crossover Rise Of The Villains, the hero Teddy dose that by betraying the other heroes by brainwashing Ms. Marvel into his minion and joining Darth Vader. also Ms.Marvel herself kinds of dose this as well do to messing up every time as Captain Marvel, She decides to become Ms.Marvel again and tries to find ways into removing the currant Ms. Marvel,after Teddy turns heel, he asks her if she would liked to be a villain and she replays with "I sometimes feel like, i am a villain" and once he hypnotizes her into a minion, there are hints that this maybe More Then Mind Control
Justain did one before Teddy, all thow it may not be his fault, do to him being a robot, who has a good and bad button on him.
Pink X started out as a good-heart girl, who wanted to be the hero, but after witness her parents murder, she took a dark path by selling information to villains about other heroes.
Drimartire did his before all of the above, after becoming a ghost and soon went to work for the likes of Anubis and Darth Vader.
Micheal started out neutral, but was still at least a good guy, until he decided to attack the heroes
Outs started out as a good cat able to talk and walk on two legs thanks to Teddy, would sadly betray him , to Darth Vader because he wanted to know how to make Morgana Macawber fall in love with him, Healx as well like outs betrayed Teddy, just because she wanted to do the same thing outs did.
Morgana herself dose this after Darkwing Duck leaving her during a battle on Bad Guy World(beliving that she had died during said battle) joins Darth Vader becoming his 6th in command.
Happens in Some Semblance of Meaning, when Phlox allies herself with Vale, Fen, and Lark, only to attempt to murder them in their sleep. Thankfully, this fails. However, Vale thinks to herself later on that, in spite of her actions, Phlox wasn't really evil; it wasn't the right thing to do, but "this is the Hunger Games. We all do things that we don't want to do—like killing and stealing and betraying—because let's face it: every single one of us wants to escape this sadistic prison the Gamemakers set up for us, to make it home alive. To our friends and families. For that, we would do anything."
Tambryn in Claustrophobia started off as a nice, well-liked character. Then she went off the deep end and tried to crucify her ally and district partner Maxim, who was only thirteen.
Same as in canon, Princess Luna becomes Nightmare Moon, and provokes the Equestrian Civil War out of jealousy of her sister. This is also expanded upon in the fic to also be due to her massive unpopularity and her thinking that Celestia no longer loved her any more.
In a smaller example, the pegasi war hero Thunderhide also joins Luna's side after losing faith in the Celestia-Equestrian system, and feeling that his bretheren's sacrifices in the war were in vain.
Trixie starts out the story as The Atoner, who wants to learn true magic from Twilight in order to make up for her boastful and arrogant past. When Twilight turns, she talks Trixie into helping strike at the other Elements of Harmony... then Trixie sees how powerful Twilight is, and performs a full turn herself in order to convince Twilight to keep her.
In the Pony POV Series, Fluttershy suffers a severe Break the Cutie and falls victim to the same corruption as Nightmare Moon, transforming into the Big Bad of the story arc, Princess Gaia/Nightmare Whisper.
The Nameless Passenger turns out to be a version of Twilight who turned into a vengeance crazedNightmare seeking to punish Discord for all eternity via "Groundhog Day" Loop, no matter how many lives she may ruin or erase in the process.
Horseshoes and Hand Grenades has Gentaro do one when Kengo didn't revive him. A being known as Ophiuchus resurrected him and brainwashed him to believe that his friends wanted him to die and left him to the hands of Ryusei Sakuta/Kamen Rider Meteor. Things get very bad.