[[folder: Crossovers ]]

* In the crossover ''FanFic/RiseOfTheVillains'', 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.


[[folder: Literature/The Hunger Games ]]

* Happens in ''Fanfic/SomeSemblanceOfMeaning'', when [[spoiler: 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 [[spoiler: her]] actions, [[spoiler: Phlox]] wasn't really evil; it wasn't the ''right'' thing to do, but "this is [[BlackAndGrayMorality 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 ''FanFic/{{Claustrophobia}}'' started off as a nice, well-liked character. [[spoiler: Then she went off the deep end and tried to crucify her ally and district partner Maxim, who was only thirteen.]]


[[folder: Western Animation/My Little Pony Friendship Is Magic ]]

* Many characters in ''FanFic/EquestriaAHistoryRevealed'' end up pulling this trope.
** King Sombra was initially an academic scholar with a love for learning and writing books, and turns into the snarling evil villain of canon, [[spoiler: hinted to be because he was driven insane by the [[PlayedForLaughs crystal ponies annoying him too much with their inane requests]].]]
** Same as in canon, Princess Luna becomes Nightmare Moon, and [[spoiler: 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 [[spoiler: losing faith in the Celestia-Equestrian system, and feeling that his bretheren's sacrifices in the war were in vain.]]
* The plot of ''FanFic/InnerDemons'' is driven by a prophecy that Twilight Sparkle will succumb to the [[EnemyWithin darkness in her heart]] and become the "Queen of Darkness", who will [[TakeOverTheWorld take over all of Equestria]]. This happens within half a dozen chapters, and Queen!Twilight proceeds to take up the mantle of BigBad for the story.
** Trixie starts out the story as TheAtoner, 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 [[InLoveWithYourCarnage how powerful Twilight is]], and performs a full turn herself in order to convince Twilight to keep her.
** In a rather ShockingSwerve, [[spoiler: Sweetie Belle and Scootaloo]], who join up due to a combo of MoreThanMindControl and not seeming to be fully aware of what's happening.
* In the ''FanFic/PonyPOVSeries'', [[spoiler:Fluttershy suffers a severe BreakTheCutie and falls victim to the same corruption as Nightmare Moon, transforming into the BigBad of the story arc, Princess Gaia/Nightmare Whisper.]]
** There's also the [[BadFuture Dark World]] versions of the [[BrainwashedAndCrazy Discorded]] Mane Six, who [[BigBad Discord]] uses as his CoDragons for a thousand years [[spoiler: until Cadence's spirit helps Twilight remember her true self, beginning the eventual {{Heel Face Turn}}s of all of them]].
** [[spoiler:The Nameless Passenger turns out to be a version of Twilight who turned into a [[HeWhoFightsMonsters vengeance crazed]] [[SuperpoweredEvilSide Nightmare]] seeking to punish Discord for all eternity via GroundhogDayLoop, no matter how many lives she may ruin or erase in the process.]]


[[folder: Anime/Neon Genesis Evangelion ]]

* ''Fanfic/NeonGenesisEvangelionGenocide'': When [[spoiler:Ritsuko]] is sick of being [[spoiler:Gendo's]] tool, she allies herself with [[SmugSnake Kluge]] to bring her ex-lover down.


[[folder: Franchise/Pokemon ]]

* May in ''[[Fanfic/WelcomeToTheWorldOfPokemon Sunny With A Chance Of Thunderstorms]]'' betrays Brendan and joins Team Aqua.


[[folder: Franchise/Star Wars ]]

* The whole first installment of the SonOfSunsTrilogy is an extended FaceHeelTurn for [[spoiler: Luke Skywalker]].


[[folder: Others ]]

* The entire plot of ''FanFic/QueenOfAllOni'' is kicked off when Jade's SuperPoweredEvilSide, the Queen of the Shadowkhan, is reawakened, setting Jade up as the BigBad.
** In the second half of the story, [[spoiler: Jade [[BrainwashedAndCrazy brainwashes Viper]] into being her new Samurai Khan General, Hebi.]]
* In ''FanFic/{{Respect}}'', [[spoiler:[[Anime/SmilePrettyCure Yayoi Kise]] undergoes a turn from sweet, timid, cybaby artist to berserk supervillain due to severe guilt and SanitySlippage.]]
* In the Power rangers fanfic [[http://www.fanfiction.net/s/6090657/1/Red_Poison Red Poison]] this happened in a parallel reality in which Jason switched sides out of jealousy of Tommy.
* Literature/SherlockHolmes in ''Fanfic/ChildrenOfTime'', as the combined end result of a BreakTheCutie experience and a SadisticChoice. He later tells [[AscendedFangirl Beth Lestrade]] that he was "curious". Her [[ThePowerOfLove unconditional love]] and HeroicSacrifice force him to truly ''[[HeelRealization understand]]'' what he's become and what he's done to her.
* [[spoiler: Bondís supposed informant]] in ''WebVideo/DiamondsCut'', a ''Film/JamesBond'' fan film.
* ''FanFic/HorseshoesAndHandGrenades'' has Gentaro do one when [[ForWantOfANail 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.
* Maylu in ''Anime/MegaManNTWarrior'' fanfiction, ''[[Fanfic/MaylusRevenge Maylu's Revenge]]''.
* In Fanfic/TheKeysOfTheKingdom, Organization XIII consists of those who fell to the darkness and became Nobodies. This includes [[spoiler: [[VisualNovel/FateStayNight Emiya Shirou, Matou Sakura]], and [[Manga/MahouSenseiNegima Takahata Takamichi]].]]
* ''Fanfic/MortalKombatDesperation'': The BigBad of this ''Franchise/{{Mortal Kombat}}'' fanfic, [[spoiler:Raiden]], becomes this after he CameBackWrong. He starts out as a WellIntentionedExtremist with a bit of the KnightTemplar and HeWhoFightsMonsters mentalities, but as the plot progresses, he becomes [[AxCrazy increasingly homicidal]] and [[TheSociopath sociopathic]] to the point of alienating his ex-allies when he [[spoiler:refuses to reconsider his invasion of Outworld and when he crosses the MoralEventHorizon by mass-zapping the Lin Kuei to near-death.]]
* A major plot point in ''Fanfic/OutOfTheCornerOfTheEye'' is [[spoiler: Paco making a [[DealWithTheDevil deal]] with Hsi Wu, becoming his [[DemonicPossession host]] in exchange for the power to kill the man who [[YouKilledMyFather killed his parents]]]].
* ''[[https://archiveofourown.org/works/12561552 Don't Haze Me]]'' is a ''Kim Possible'' one-shot where Kim goes down the slippery slope to becoming a villain. Instead of Shego undergoing a HeelFaceTurn and joining Kim like in other Kigo fics, ''Kim'' takes a FaceHeelTurn and joins Shego. Kim becomes a WellIntentionedExtremist who wants to make the world better by killing the "bad guys", including [[spoiler:Drakken]].