* In ''Roleplay/TheGamersAlliance'', [[SnarkKnight Abel]] makes this very clear by adapting the name Cain once he's turned to the dark side. The heroes are oblivious to his new, treacherous nature and think he's only acting emo because of his embarrassing encounter with a lustful mage earlier in the story. [[spoiler:[[HeWhoFightsMonsters Refan]] and [[CloudCuckooLander Ronove]]]] also make a FaceHeelTurn later on when they side with demons.
* [[spoiler:Corey]] from ''ThreeInTheAfternoon'' does this not once, but ''twice'', taking full advantage of his more idealistic friends the second time around.
* [[spoiler:Julia van Helden]] does this in ''KateModern'' in response all the traumatic experiences she suffers while helping the K-Team, and out of misplaced [[LoveMakesYouEvil love for a villain]].
* In ''AssociatedSpace'', [[spoiler:Nazar]] actually defeats Fatebane, captures him, and brings him to the villain. However, when he doesn't get paid, he decides to break the hero out, inadvertently joining Fatebane's side.
* [[spoiler:Reynold]], of ''WebAnimation/HomestarRunner'''s ShowWithinAShow ''The Cheat Commandos'', recently went over to work for the evil forces of Blue Laser. All things considered, it was surprisingly plausible.
* Any time a previous non-player starts to actively participate in the 'game' in ''SurvivalOfTheFittest''. As you might imagine, this tends to happen a lot.
* [[spoiler:Agent Washington]] of ''Machinima/RedVsBlue''. [[spoiler:Thankfully, it's temporary]].
** And as of Season 12, [[spoiler:Felix]]
* [[spoiler:Ephram]] from DeadEnds does this when most of [[spoiler:his]] family is killed by zombies [[spoiler:and Eddie's stray gunshot.]]
* [[spoiler:Alex]] of WebVideo/MarbleHornets does this ''somewhere'' between seasons 1 and 2 [[spoiler:by appearing to work/cooperate with the Operator]], though it's made ''explicitly'' clear in season 2's finale that he's no longer any friend to Jay and Jessica. Of course, [[SanitySlippage certain circumstances]] may have led him to that end.
* MSFHigh: The biggest example would be Gestell, due to his split personality issues, being revealed as the villain of one arc. It's not The Mole, as he had shifted away from hanging out with the players, and did not sabatoge their efforts using this.
* [[spoiler:Ma-Ti and the Cinema Snob]] in ''WebVideo/ToBoldlyFlee''.
* In ''Literature/{{Worm}}'', there are a few cases of heroes opting to become villains ([[spoiler:Accord]] and [[spoiler:Flechette]] being the obvious ones), but the most interesting case comes when [[spoiler:Scion switches from fighting for humanity to destroying it]].
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