* If taken to extremes, it can result in a case of RonTheDeathEater.
* In Fanfic/KeepersOfTheElements, Courtney starts off as her usual bossy, {{Jerkass}} self, but then goes up OverNineThousand levels when she [[spoiler:[[DealWithTheDevil makes a deal]] with [[BigBad Radcliffe]] for powers of her own so she can get revenge on [[TheLeader Gwen]]]]
* In the ''Franchise/{{Disgaea}}'' fanfic [[http://www.fanfiction.net/s/6984913/1/Disgaea_Jewel_of_the_gods Disgaea: Jewel of the gods]] Laharl is a HUGE JerkAss to Adell, and Mao not even caring if they died, while in canon, he has a HiddenHeartOfGold for people.
** Well to be fair, [[MoralityChain Flonne]] isn't there most of the time, making him a complete asshole to most people.
** He also REALLY hates Adell and Mao for being main characters in the sequels to Disgaea.
* In ''Fanfic/HerdingCats'', ''Tavros'' of all people becomes a huge dick after being successfully shipped, particularly towards Equius, Sollux and ''especially'' Aradia [[spoiler:to the point where the two of them become kismeses]]. It happens as a result of his becoming [[spoiler: Vriska's]] moirail, because she helped him become confident enough to overcome his own problems and be more assertive, but neither realised how over the top he was being. Everybody else has to point out that there's a difference between countering light insults from others and being absolutely horrible to them. They also have to point out that anyone who does something as horrible as blaming ''Aradia'' for his spine getting broken and tries to play it off as being assertive seriously needs a reality check. He gets [[WhatTheHellHero put through the wringer]] for it.
** This actually happens to more than a few people. [[spoiler: After Eridan and Terezi get together,]] Karkat flips his shit and goes off on an angry rampage that lasts for most of the fic. In turn, Terezi continually throws horrible barbs at him, coming off as a huge bitch herself. Later, Tavros rubs off on Gamzee, who continually treats Equius like shit.
* Literature/SherlockHolmes in ''Fanfic/ChildrenOfTime'': the series is really about [[CharacterDevelopment his development as a person]], from good to better to worse to worst to best. Yes, really. And after having seen him as a lighter, happier Holmes early on in the series, it's really tear-jerking to watch [[BreakTheCutie that lighthearted-ness break down]] until he's outright emotionally abusing the KidHero of the story. [[FromBadToWorse And it only gets worse]] [[FaceHeelTurn from there]].
* Moonpaw in the ''Literature/WarriorCats'' fanfic, ''FanFic/MoonlitPathDisloyaLHearts''. She starts out spunky with a bit of sass. Later on, she becomes a selfish jerk. Her sudden dip in niceness is more understandable when you consider that she started going bad right after [[spoiler: she receives a dream predicting her death.]]
* As [[Manga/DeathNote Sayu]] points out in ''Fanfic/TheFaceless'': "Everyone used to love my brother. Now, he is [[JerkAss a complete asshole.]]"
* The now adult Takeru in the ''FanFic/TamersForeverSeries'' is much more blunt and cynical than he was as a child. The fact he used to hold the Crest of Hope makes this darkly ironic no less.
* Maylu from ''Anime/MegaManNTWarrior'' fanfic, ''[[FanFic/MaylusRevenge Maylu's Revenge]]'', has become like this, lashing out at [[NeverMyFault Roll for making her unable to continue in the N1 Grand Prix after turning her back to normal from being Empress]], and [[ItsAllAboutMe not being able to become a strong [=NetBattler=]]]. This would lead her into defecting to World Three and attempting to delete Roll as revenge for the moment.
* In the story ''FanFic/SylviaTheSylveon'', Alseides and Vapor, two of Sylvia's brothers, the former due to the disappearances of two of his other brothers (both of whom, coincidentally, TookALevelInKindness) and later, the [[BloodSport illegal fighting]] for which he is trained after he himself is kidnapped, and the latter due to both being mistreated by his trainer and not getting the evolution he wants.
* Navarone becomes increasingly unpleasant over the course of ''FanFic/DiariesOfAMadman'', though this is acknowledged in-universe and also justified due to the bad stuff that happens to him over the course of the story. However, he does improve on this in a few areas as well.
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