* Anyone that breathes in the Giygas gas in ''FanFic/AttackOfGiygas''. As for named characters, we have Dr. Mario, Pichu, Ashley (who claims that she's sane), [[spoiler: Lucas, and Link.]]
* Red Cyclone, Night Raven and (to a lesser extent) [[spoiler: Gilda]] in ''Fanfic/AceCombatTheEquestrianWar''
* ''Fanfic/BatmanBeyondRevisited'': The appropriately named OC, Chainsaw, a leader of a Jokerz gang that thrives on torture and murder to the point that the Joker himself would be disturbed.
* Pinkie Pie in ''FanFic/{{Cupcakes}}''.
* In ''ComicBook/{{Supergirl}}'' fic ''Fanfic/HellsisterTrilogy'', Satan Girl is a Supergir's duplicate, the embodiment of Kara's dark side with no conscience or restrain to control her murderous impulses.
-->Kryptonians could survive in space without a suit. Was that not a pleasure? It certainly was. She could live her life between the stars, and never once need to breathe.\\
She could devastate planets, wipe them clean of life. Rebuild them at her whim.\\
She could tyrannize worlds, whole systems of planets, make them bow to her mighty hand, instantly execute anyone who dared protest--or just anybody she wanted to kill.\\
She could explore pleasures of the body that Kara never would have dared to, satisfy lusts that the blonde beast never even knew she had. She could force herself upon any suitor, male or female or whatever, and destroy them after their job was done. Or perhaps just maim them, so that they could never again do such a job for anyone else. Satan Girl smiled. Now that was being imaginative...\\
She could have children from those couplings, or kill them in the womb.\\
She could become a goddess to an unsophisticated planet's people. Drinking in their worship, demanding sacrifice.\\
All of this she could do, she would do, and more.\\
For Kryptonians and Daxamites were gods, off their homeworlds. They really were. What a pity their morality forced them not to realize that fact.\\
She clasped her bent knees to her chest and thought. The problem was, in this time, she was hardly unique. Billions of Kryptonians existed on Rokyn. Billions more Daxamites, with the same power, existed on Daxam. Luckily, there was only one prisoner still left in the Phantom Zone, that old poop Gazor, so there wasn't much competition there.\\
But, somehow, she'd have to do something about both planets. Daxam would be easy. A shower of leaden hail across its surface, and the dead would litter the ground in heaps beyond Hitler's and Stalin's dreams.\\
That world would stink of corpses for eons to come.\\
She laughed soundlessly.
* [[Manga/DeathNote Soichiro Yagami]] in ''FanFic/LightAndDarkTheAdventuresOfDarkYagami'', holding a knife to Mikami's neck "like the Joker".
* [[spoiler: Daemon]] from the ''FanFic/TamersForeverSeries'' could be the posterboy for this trope, a nigh unstoppable psychopath who is ''fuelled'' by his own hatred, often exploding into violent rages and laughing maniacally at the sight of others in pain.
* Joe Dark from ''FanFic/ClashOfTheElements'' is a very prime example of this trope [[spoiler: and this very trait of his is amplified immensely after he gains the Dark King's powers.]]
* Captain June Harper, VillainProtagonist of the ''Series/DoctorWho'' story ''{{FanFic/Gemini}}'' is a SerialKillerKiller on the run from the Time Agency that she used to work for. She maims, tortures, and murders anybody she that deems to be threatening the innocent, and she takes great pleasure in the pain and fear that she inflicts on her victims.
* ''FanFic/TheImmortalGame'': Nihilus, Twilight Sparkle's SuperpoweredEvilSide, is absolutely batshit crazy.
* Similarly, in ''FanFic/InnerDemons'', after Twilight is consumed by her [[EnemyWithin Queen persona]], she becomes an extremely mentally unstable sociopath who [[MoodSwinger switches]] between cold emotional detachment and blind fury at the drop of a hat. It gets to the point where she even decides to kill her brother when he rejects her [[VillainousIncest advances]].
** And that's ''before'' her VillainousBreakdown. Afterwards, she's left constantly switching between laughing maniacally and sobbing (it's actually implied that's her true personality trying to resurface and being forced back down by the Queen), and on top of that she's [[spoiler: decided that rather than ruling Equestria, she'll [[OmnicidalManiac destroy it]]]].
* ''FanFic/AShadowOfTheTitans'': ( A ''WesternAnimation/JackieChanAdventures'' and ''WesternAnimation/TeenTitans'' crossover fic) [[PsychoForHire Gadjo]] is completely psychotic, to the point that [[EvenEvilHasStandards even the rest of the HIVE find him disturbing]]. He tends to be paired up with the similarly crazy villainess Machete.
* ''[[FanFic/MyLittleMagesTheNightmaresReturn My Little Mages: The Nightmare's Return]]'' has resident former Wonderbolt turned captain of the Shadowbolts [[DarkActionGirl Nightshade]]. She killed several of her fellow Wonderbolts just because Spitfire was [[ItsAllAboutMe named captain over her]], and only agreed to join Nightmare Moon when promised that her job would be [[PsychoForHire to kill people]]. When trying to [[WeCanRuleTogether recruit Rainbow Dash]], she even encourages her to be a BastardUnderstudy, fully expecting to one day be killed by her, and sees nothing odd about that.
** [[spoiler: When Pinkie is exposed to the [[PsychoSerum Heart's Desire]] by [[TheDragon her father]], she basically turns into her ''FanFic/{{Cupcakes}}'' self. Fortunately, she is just barely able to [[FightingFromTheInside retake control]] before killing Twilight.]]
* The ''Fanfic/PonyPOVSeries'' has Dark World!Fluttercruel. Being [[{{Foil}} what normal Fluttercruel might have been if she had been raised by Discord]], Fluttercruel is a sadistic PsychopathicManchild who gleefully [[PsychoForHire does Discord's bidding because she loves killing ponies]]. Her default reaction to something frustrating her is basically 'pull out large, deadly weapon, and slice target of frustration in half'. Oh, she has a '[[TortureCellar playroom]]' where she basically reenacts ''Fanfic/{{Cupcakes}}'', complete with cannibalism. She's ''so'' AxCrazy even ''Discord'' is trying to get her to mellow out...[[PragmaticVillainy because he considers outright murder boring and doesn't want to run out of toys.]]
* [[spoiler: Nightmare Phobia]], the BigBad of the ''[[Fanfic/PonyPOVSeriesChaosVerse Chaos Verse]]'' (an official spinoff of the ''POV Series'' above) is this trope in spades. And it's not helped at all by the fact that the more powerful she becomes, [[TheDarkSideWillMakeYouForget the more her memory is corrupted]], which in turn only fuels her insanity more.
* ''FanFic/AGrowingAffection'' has the self-named Reaper of Shinobi. She thinks that anyone who uses chakra (herself included) is a demon, so her goal is to kill all ninjas and then kill herself. Over the years she has come to believe the backstory she fabricated for herself (that she is the sole survivor of the destruction of her village in the last war).
* Several characters in ''FanFic/YouGotHaruhiRolled''. The fic follows canon in portraying [[KnifeNut Ryouko]] in this way, but in addition, Haruhi is occasionally [[{{Flanderization}} Flanderized]] into this trope, such as when she threatens to kill Kyon's sister for dishonoring the SOS Brigade. Additionally, Sasaki usually functions as the OnlySaneWoman of her group, but in the chapter where she finds a Manga/DeathNote, she arguably out-crazies [[AGodAmI Yagami]] [[VillainProtagonist Light]] himself.[[note]]Though this is only implied, rather than outright stated.[[/note]]
* Twilight Sparkle is the prime example in ''Fanfic/PagesOfHarmony'', though Rainbow Dash is definitely no slouch in that regard either. Twilight basically tortures and [[MindRape Mind Rapes]] her friends to extract their Elements, becoming increasingly deranged the further she goes down the road. Rainbow Dash, the first killed as a result of Twilight's actions, has her memory twisted and warped to the point where she [[spoiler: [[ImAHumanitarian cannibalizes herself]] as a result of no longer having Loyalty to even herself]], and swears that she'll find a way to kill Twilight for what she did.
** Not to mention, as is revealed during [[spoiler: Fluttershy's]] extraction session, Twilight intends to [[spoiler: "cleanse" the universe (and the ''multiverse'' if it exists) by [[ANaziByAnyOtherName destroying all living beings without the genetic capacity for the Elements of Harmony]]]]. And she's convinced in her mind that [[IDidWhatIHadToDo it's all]] [[UtopiaJustifiesTheMeans for the greater good]].
*** Speaking of [[spoiler: Fluttershy]], she reaches AxCrazy UpToEleven after [[spoiler: Twilight's completely extracted Kindness]]. She kills [[spoiler: her pets]] in a violent fit of rage, tearing them apart into bloody messes with no regard for any of their lives, before she [[spoiler: attacks Twilight and wounds her, the latter LaughingMad all the while and trying to fight back]] before [[spoiler: Fluttershy]] dies. And she does this while having [[BodyHorror wounds from being sliced apart]], [[KillItWithFire burns from various procedures]], [[EyeScream one eye destroyed]], and ''[[BodyHorror half her limbs missing from chemical use]]''.
* Professor Hojo Okokou in Sephirothslave's ''Fanfic/ShinraHighSOLDIER'' definitely counts. He gets more and more violent and crazy as the story progresses.
* Hitomi Kirihara in ''Fanfic/PerfectionIsOverrated'' is a [[DeconstructedTrope deconstruction]] [[ParodySue of]] a JerkSue who sees nothing wrong with using MindControl to steal from her victims, force them to commit unspeakable crimes, and then make them kill themselves. [[EvenEvilHasStandards Her fellow SUEs are disgusted by her behavior]], partly [[PragmaticVillainy out of justified fear that she will turn on them]].
* [[VideoGame/{{Yume2kki}} Urotsuki]] is portrayed as this in Creator/HyperInuyasha's stories. She's completely murderous toward anyone that she doesn't like, and even if she likes you, it doesn't guarantee your safety. [[EveryoneHasStandards The other characters]] [[TheDreaded rarely like her.]]
* Dalek Caan in the ''Fanfic/ChildrenOfTime'' version of [[Series/DoctorWho "The Daleks in Manhattan"/"Evolution of the Daleks"]]. ColdBloodedTorture, RedemptionRejection, & etc. As a result, he's [[HoistByHisOwnPetard hoisted by his own petard]] in the end.
* Two examples in ''Fanfic/TheLionKingAdventures'':
** [[spoiler: Scar, after going completely insane in ''Friends to the End''. He slaughters his hyena minions, including Ed, and [[MurderersAreRapists rapes an innocent teenage lioness]] called Uzuri.]]
** [[spoiler: The Writer in ''The End'', who becomes hell-bent on destroying the cubs. He even transforms into a [[SuperMode petrifying combination of all the previous villains]] just to kill Simba.]]
* [[AllLovingHero Gentaro Kisaragi]] and [[AloofAlly Ryusei Sakuta]] get onto this in ''FanFic/{{Horseshoes and Hand Grenades}}'' for different reasons: Gentaro was killed by Ryusei and [[CameBackWrong resurrected by Ophiuchus]] because he was supposedly left to die by his friends. Ryusei, [[spoiler: was brainwashed by Emoto and then reconfigured into a cyborg with the sole mission is not yet revealed.]]
* "Fanfic/BlindMansBluff" has [[IronicName the Angels]], who are anything but angelic.
* In ''[[http://tvtropes.org/pmwiki/pmwiki.php/FanFic/SinfulBehaviors Sinful Behaviors]]'' , Rudy becomes this when he [[spoiler: is taken over by the black chalk.]]
* [[Manga/{{Naruto}} Utakata]], a.k.a. Saiken the Slug, in ''Fanfic/KitsuneNoKenFistOfTheFox'', is this. In fact, before he underwent the training that gave birth to the [[PersonOfMassDestruction Kyuushingai]], he was a homeless man with serious psychotic issues, but he had the presence of mind to hide that fact from the doctors and scientists who worked on him and the other eight members of the group. The end result...
--> '''Utakata:''' They allowed a psychopath like me to be transformed into a SMART psychopath.
* Viktor Kraber in ''FanFic/TheConversionBureauTheOtherSideOfTheSpectrum''. He's [[TheAlcoholic an alcoholic]], has committed multiple acts of terrorism up to and including mass murder, and displays utter glee at the prospect of murdering the TCB!Elements of Harmony with his {{BFG}}. Even worse? He's one of the good guys, and a solid product of LoweredRecruitingStandards. {{Justified}} in that he's ''very'' deeply traumatized due to [[AdultFear having lost]] [[PapaWolf his children]] to TCB!Pinkie, and the war against TCB Equestria has broken almost every other surviving human in some way or another. Plus his BloodKnight nature is kept strictly towards targeting his enemies, and he does have [[MoralityPet Aegis]] to help keep him in check.
* In ''FanFic/MegaManDefenderOfTheHumanRace'', Dr. Wily and several of his Robot Masters are this. Both Wily and [[spoiler:Bass]] love the sound of screaming, and Splash Woman goes berserk after Roll attacks her and threatens to destroy the entire cavern to get rid of her enemies.
* In ''Fanfic/SunsetOfTime'', the BigBad, Vesper Radiance. She's an insane psychopath who has personally murdered several characters already, attempted to kill even more, and her victims include [[SelfMadeOrphan her own parents]] out of revenge for putting her in an orphanage as a foal.
* Craig Tucker in ''FanFic/LikePinningButterflies'' murders six people (eight if you count that he [[TheyKilledKennyAgain killed Kenny three times]]) and feels absolutely no remorse. He also kidnaps, tortures, and almost kills [[AbductionIsLove his crush]].
* ''FanFic/RosarioVampireBrightestDarkness'':
** Tsukune's ghoul form counts to a T. In Act III chapter 52, when unleashed, he proceeds to brutally dismember and disembowel Kiria with his bare hands, [[EvilLaugh laughing]] [[PutTheLaughterInSlaughter while he does it]].
** Jovian and Jacqueline Kikion also qualify, particularly the originals who served Hokuto. In Act III chapter 52, when Hokuto warns Felucia that Jovian will kill Ruby and Tsukune if Felucia won't hold them back, Jovian gleefully cheers that she loves killing. The resummoned Jovian and Jacqueline in Act VI are considerably more mellow.
** Arial Kuyumaya often comes across as this in Act VI, in part due to her HairTriggerTemper and obsession with Dark; in chapter 7, the ''very'' first thing she does as soon as she discovers that Mizore is Dark's girlfriend is use her ice claws to literally tear Mizore apart in a jealous rage ''right in front of everybody''; by the end, Mizore is so horrifically injured that Ceal has to transfer organic matter from deceased [=HDA=] officers to actually heal her. The others even outright describe her as insane on more than one occasion.
** Even without chrono dementia, Evil!Falla is a sadistic, sociopathic bitch who can slaughter people by the handful without hesitation, while laughing about it.
** As revealed in Act VI chapter 52, the Fairy Tale-created clone of Moka is little more than a savage animal.
* ''Fanfic/RottingCamellias'' has Mayu, a knife-thrower in a circus who seems to be this.
* The ''Franchise/TheLegendOfZelda'' fic ''[[FanFic/BoundDestiniesTrilogy Wisdom and Courage]]'' features Veran, who ''clearly'' enjoys being a destructive, mass-murdering bitch far more than she desires to TakeOverTheWorld, to the extent that her ''very first thought'' upon arriving in Clock Town was to blow up the city and be done with it. Link even outright [[YoureInsane calls her insane]] on multiple occasions.
* In ''FanFic/PrisonIslandBreak'', Shadow is a completely unpredictable psychopath. It's for a good reason though; [[spoiler:he was locked in the Hole with Scourge after attempting to escape, Scourge basically being Ax Crazy and NightmareFuel given physical form.]]
* ''Fanfic/HopeForTheHeartless'' includes as minor antagonists a pack of more than thirty SavageWolves, fittingly called the Mad Pack. They have terrorized the wildlife and farmers of [[Disney/TheBlackCauldron Prydain]] for the last twenty years, increasing in numbers over the years. They suffer from an incurable disease similar to rabies, which has robbed them of all reason, fear and natural instincts. They live only for killing, rarely touching the meat of their killings. When they get the taste of blood, ''nothing'' can discourage them from the hunt. They attack and kill ''anything'' that moves -- big cats, bears, entire wolf packs, any forest animal they can sunk their teeth into, livestock and humans (even a human village). Heck, they even attack ''[[VillainProtagonist the Horned King]]'' who strikes [[WalkingWasteland fear in every living creature]].
* ''Fanfic/PacificWorldWarIIUsNavyShipgirls'' has [[TokenEvilTeammate Phoenix]], who is also a PyroManiac.
* ''FanFic/SonicXDarkChaos'' has Tsali. While moderately calm and [[FauxAffablyEvil even somewhat civil]] when he's not in battle, all bets are off in a fight. His greatest joy is murdering Seedrians in the most gory ways he can think of; everyone else is fair game too. And [[SanitySlippage as the story goes on]], the "Crazy" part comes to the forefront.
** The Muslims are basically an entire faction of violently insane pedophile fundamentalists.
* ''Fanfic/NoahMaxwellTakesALittleTripHere'' has both an heroic and villainous example in [[spoiler: Noahbrand and Patrick Andersen.]]
** When [[spoiler: Noahbrand]] appears for the first time he literally [[spoiler: sears his ''handprint'' into Patrick's neck]] and seems to take unholy glee in [[spoiler: watching Patrick burn.]] He does seem to be mostly ChaoticGood, though, seeing as he [[spoiler: spared Patrick after learning about Michael.]]
** [[spoiler: Patrick,]] however, is a straight-up villainous example. He [[spoiler: stalks Evan,]] seems to have creepy FoeYay tendencies towards everyone in Team CTNE save Caught, and is quite frankly even creepier than he was in his [[spoiler: [[WebVideo/MLAndersen original work.]]]]
* In ''Fanfic/ThousandShinji'', Asuka is a rare heroic -or rather anti-heroic- example. After becoming a worshipper of WarGod ''[[TabletopGame/{{Warhammer 40000}} Khorne]]'' she was gradually turned into a psychopatic, blood-thirsty, axe-wielding berserker. However, although she went on a killing spree, she exclusively targeted thugs, rapists and drug-dealers, and she killed them because she was trying to save her boyfriend.
* In the opening pages up ''[[https://www.fanfiction.net/s/9678557/1/Uplifted-Arrival Uplifted: Arrival]]'' Joachim Hoch [[spoiler: ties noted real life Nazi War Criminal Oswald Pohl to his bed posts and proceeds to behead him with an axe. He then has the head sent to Berlin as a warning.]]
* The ''Literature/ASongOfIceAndFire'' fanfic ''FanFic/RobbReturns'' has Viserys Targaryen, [[spoiler: especially after he is given a huge (fake) dragon egg.]]
** [[spoiler: In Chapter 70, he completely snaps and attempts to awaken his dragon by murdering one of Illyrio Mopatis' servants and then his own sister.]]
** Readers get to see into Lysa Arryn's mind [[spoiler: after she (nearly) murders Jon.]] Mind-scattered is the least of the problems she has. As one reader commented, "Lysa's mind gives me cancer".
** [[spoiler:The ancient Valyrian who calls himself 'the last Dragonlord', who commands a host of [[BeastMan beast-headed men]] and is trying to capture a feral dragon.]]
* [[spoiler: Mania]] from Fanfic/TalesOfAResetMind is this mixed with {{Determinator}}
* [[VideoGame/{{Overwatch}} Reaper]] isn't all that sane in Overwatch canon, but ''[[FanFic/BreakMyHeartBreakYourHeart Break My Heart, Break Your Heart]]'' turns his instability and murderous tendencies UpToEleven. Nearly all of his scenes in the fic ''far'' surpass the game's usual levels of violence, and he is ''[[NightmareFuel terrifying]]''.
* ''Fanfic/ForumOfThrones'' has Wolfius Woodbark, the psychopathic serial killer introduced early in Book 1. Over the course of the story, he shows absolutely no hesitation to act with utmost brutality, sometimes without any provocation at all. He is so unstable that his companions outright ditched him to make sur that he won't compromise their mission.
** Kreep has signs of this as well. Most of the time, he shows no emotion at all, but his eagerness to get into a fight and the brutality he uss to dispatch his victims is unsettling, to say the least. [[spoiler:Fittingly, he is revealed to be the son of Wolfius]]
** Alysanne Waters is another example. She is as unstable as she is dangerous, a psychopath with a violent temper. One scene that sees her fighting against several men at once has her outright laughing at the violence around her.
* ''Fanfic/TheMakingsOfTeamCRME'':
** [[Fanfic/TheBlackHearts Marcus Black]] is a sadistic assassin and the abusive father to Mercury (who calls him a psychopath several times). Marcus specifically [[SociopathicSoldier joined the Atlesian military just to kill people]]. And when he was discharged from that profession, he went to being a ProfessionalKiller [[PsychoForHire to sate his bloodlust]]. He enjoys fighting people and has violent outbursts often that he [[DomesticAbuse takes out on Mercury and his wife Melanie]]. He's also an avid torturer and will even take extra pay if someone wants the victim to suffer. [[spoiler: His ax-craziness comes to the forefront when he attempts to kill Mercury.]]
** [[Fanfic/RomansEmpire Neopolitan]] is [[CharacterExaggeration even more sadistic in violent in this series than she is implied to be in canon]]. She takes a lot of pleasure in killing people for Roman and likes cutting people up. Even when Roman tells her to stop, she still has to get one last stab in. Neo [[MurderIsTheBestSolution will always suggest killing people whenever Roman has a problem]] and gets let down whenever he says it won't be necessary. Roman lampshades this by saying that she'd likely be a serial killer if she didn't work for him.