The result of shoehorning a good canon character into being a villain or making a villain significantly more evil than in canon is Ron the Death Eater, the inverse of Draco in Leather Pants.
This can be seen as a kind of deliberateFlanderization — Often, in creating Ron The Death Eater, a fanfic writer spins his canonical (non-evil) actions into evil acts, and every possible negative trait of the character is exaggerated. A measure of ruthlessness becomes complete and utter sociopathy, a tendency towards holding grudges becomes an obsessive hatred of anything they dislike, slight denseness becomes raging stupidity, etc.
This is often the result of also having a Draco in Leather Pants, but it doesn't have to be - some characters inspire this sort of hatred on their own, either by their canon merits or by being a romantic rival that interferes with the writer'sOne True Pairing, especially if the subject is part of an Official Couple. Or maybe they're getting in the way of the author's Mary Sue.
In other cases, it seems like the target character, regardless of their original characterization, will simply default to one of two stock personalities, based entirely on their gender. Basically, male characters end up as drunken assholes while female characters end up as manipulative bitches. In any case, Ron the Death Eater is more likely to be a Card-Carrying Villain who does things For the Evulz than to have any plausible reason for switching sides.
The fic may also have other characters who are canonically friends of the victim act as though he or she has always been an object of justified loathing, rather than going the "shocked at betrayal" approach. Or, even if these characters end up siding with the Draco in Leather Pants, they are subject to some character defamation themselves by way of "I was stupid to love him and not you".
Named for the tendency in Harry Potterfanfics where Draco Malfoy turns good and hooks up with Hermione to have Ron — in canon a decent, upstanding sort of fellow firmly on the side of good who happens to have a long-standing enmity with Draco — lose his mind and often join Lord Voldemort just for a chance to kill the sainted Malfoy.
A Sub-Trope of Demonization.
Compare Die for Our Ship or Derailing Love Interests (the major cause of this trope), Designated Hero (the other major cause), Historical Villain Upgrade, Dark Fic, and Adaptational Villainy.
Contrast Draco in Leather Pants.
And last but not least, fandoms that have so many examples of this that they needed to be moved to their own pages:
The entire League of Extraordinary Gentlemen seems to be based around this trope. Mina, who at the end of Dracula was happily married to Jonathan Harker, divorces her husband and presumably abandons her child prior to the beginning of the first volume. The upstanding Great White Hunter Alan Quatermain is an opium addict. Perhaps the most extreme example is from Black Dossier. The comic features a violent, sociopathic version of James Bond in a Lawyer-Friendly Cameo. Jonathan Harker goes from a loving and fiercely devoted husband to dumping his wife for being 'soiled goods'. In LOEG 2009, Harry Potter is turned into The Antichrist.
Mary Jane from Spider-Man is a frequent victim of this In-Universe. Basically, whenever a writer wants to break up MJ and Peter they'll try to do so in a way to make Peter look like a victim so he can angst over it. First time was her rejection of his proposal and shallow break up with him (though while MJ was yet to finish her character development and was a bit shallow, it was STILL very out of character for her to be so unapologetic about it), second time was serious bitchification before then killing her off (didn't last, fortunately), third time was, AFTER they already broke up, they decided to have One Moment In Time to explain that it was all MJ's fault, and wrote her as an emotional trainwreck who was ready to break down from stress. Thankfully, only some fans take these at face value and blame MJ for the bad writing.
TRON: Ghost in the Machine (a mini-series of comics based off the Alternate Continuity of TRON 2.0) depicts Alan as somewhat callous, work-driven, and harsh towards his son. Of course, his circumstances in the 2.0 timeline suck worse than his circumstances in the TRON: Legacy one — Flynn still vanishes, but he's been relegated to a lab instead of the boardroom, and while Lora's alive in Legacy, she died in 2.0 — but the depiction really goes off the rails when he was shown murdering his wife in a jealous rage. Granted, it was just a villain's attempt to Mind Rape Jet, but still.
Parodied in The Unwritten, about a son of a writer who got famous after creating a series of novels very similar to Harry Potter. The first two start with fragments of those books, and the third with a fragment of Frankenstein. The first page of the fourth issue shows Harry's counterpart slaughtering Ron and Hermione's counterparts in a really terrible way, only to have it turn out on the next page that it's a Dark Fic.
This even worked its way into modern cinema! While the film Troy claimed to draw most of its inspiration from The Iliad, most of the Greek characters were Flanderized into selfish and thuggish villains. Agamemnon, the pragmatic if prideful leader with a healthy dose of Big Brother Instinct for Menelaus, was reinterpreted as a tyrannical conqueror who only used Helen's elopement with Paris as an excuse to conquer Troy for himself. Menelaus, who loved Helen so much he was willing to forgive her for running off with Paris and starting the whole war, was presented as a piggish, cheating jerk who deserved to have Helen run off on him. That's not even getting into how Patroclus was almost unrecognizable from the original myth where he was the kind, compassionate companion of Achilles who only joined the battle to rescue his friends, whereas in the film he was a callow, childish brat who foolishly got himself killed to anger Achilles to get out of his tent and rejoin the war.
Vergil and Ovid both portray the Greek heroes at Troy mostly in terms of their post-victory atrocities.
Medieval European writers usually knew Latin but not Greek; as a result, they tended to inherit the Romans' bias. Dante places Odysseus in the eighth circle of Hell, and both Chaucer and Shakespeare are merciless to the Greeks in their versions of Troilus and Cressida.
Borderlands 2 DLC Tiny Tina's Assault on Dragon's Keep centers around a tabletop RPG run by the titular Tiny Tina that loosely retells the story of the main game. In it, Angel is made out to be the villainous Sorcerer's Daughter who screams lines about how everything is her fault since Tina blames her for leading Roland to his death. In actuality, Angel was an unwilling pawn of Handsome Jack (her father) and genuinely tried to help the heroes in the second half of the game. Lilith at least tries to tell Tina that the real Angel wasn't as bad as she thinks she is.
Happens In-Universe in one episode of Samurai Jack - Aku attempts to win over large group of Jack-worshipping children by telling them stories and painting himself as a hero, and when it fails he starts telling stories about Jack, portraying him as Ax-CrazyJerkass who gets rightfully deserved punishment.
Messy Nightmare is about a dream that Sakura has, in which she, Naoko, Chiharu, and Rika are all tiny inside Meilin's house and attempt to escape an Ax-Crazy Meilin as she kills each of them one by one.
Speaking of, Light and Misa try to kill Erin, an innocent girl who confronts them just as Light is in the process of writing his own father's name in what he thinks is Misa's notebook. It's assumed that Light might have gone on to kill everyone else afterwards had he not been thwarted. From their standpoint, it was probably justified, but still. Actually, it might be accurate to say that for L, Light and Misa, this trope along with Draco in Leather Pants are zig-zagged with each other.
Here's another fic where Soichiro becomes an Abusive Parent: What Is Justice?, in which Souichiro regularly beat Light and Sayu throughout their childhood. It's also implied he had their mother killed. This fic makes another Ron The Death Eater with the surprise plot twist where sweet little Sayubecomes Kira.
Pain is Pleasure by the same author as Teenage Drame, has Vegeta sense great evil in Yamcha.
Elfen Lied: In Heirs Of The Generation, an Elfen Lied and Death Note crossover, it RetCons it so Yuka was the one that alerted the facility to Lucy's location in an attempt to have her killed, and when Lucy survives getting shot up, Yuka doesn't tell anyone in the hopes she will just bleed to death or starve to death, and when that fails, Yuka becomes a fervent Kira worshipper and prays that Kira will take care of her problem for her.
Boys Do Tankary, found here, has one with Ami who, unlike many examples, doesn't have much of a fandom or hatedom. Initially set up as a Butt Monkey when the male main characters force their way into her class and rub her defeat and being passed over for a promotion in her face, she decides to get revenge against them by getting them transferred for having relationships with the girls, but ends up getting demoted to Sergeant Major. Blaming the girls, she holds them hostage and beats them up, until the boys arrive, rescue the girls and arrest her. At this point, she reveals she is working for the apparent Big Bad, "The Orchestrator," who wants to start another war, after which she is knocked out and has not been seen since.
From the same fic, and a few others Maho is depicted as an Aloof Big Sister who pushed Miho away for not living up to the Nishizumi family's standards for tankery. This is not always done out of deliberate malice, as many writers don't know that in the Little Army prequel manga it is revealed that Maho actually loves Miho dearly, and serves as Nishizumi heiress so that Miho will be free to choose her own path in life.
When Noizi Itou (the lady who illustrates the light novels) made an art of Haruhi◊ to be auctioned in support of the people affected by the Kanto Earthquake, a rabid Haruhi hater actually whined about it. Their argument was that, if anything, Haruhi would have wished for the earthquake to happen in the first place, just so she could be the star of a movie consisting of actual, unfilmed events. (The real Haruhi does not like destruction, and the Closed Space exists specifically to give her an outlet to vent her frustration without causing any damage in the real world.)
The fancomic Gulliver depicts Haruhi as a bikini-clad giantess who stumbled upon an island containing a foot-tall city and decides to go on a stomping spree. Though, that might be about more than character assassination.
In Chunin Exam Day, the other members of Team Seven aside from Naruto get hit with this extremely hard, to the point where Kakashi kills Naruto for embarrassing Sasuke, indicative of a common trend of fanfic authors accusing him of favoring training Sasuke to Naruto and Sakura's detriment. It's a "Groundhog Day" Loop situation, so he gets better, but still.
Naruto One Man Team has this with multiple characters, especially Sasuke and Sakura, with Sasuke attempting and threatening rape, attempting to rob/murder Naruto, and more while Sakura agrees with everything Sasuke does.
In Team8, Hiashi gets this quite a bit. In canon, he looked down on Hinata and saw her as weak, but gradually came to respect how far she had come; in this fic, he not only tells her that she is weak to her face, but he also physicallyabuses her. Another stark contrast point is how he treats Neji; in canon, after Neji loses to Naruto, Hiashi goes up to tell Neji the truth about his father, ending by prostrating himself before Neji and begging his forgiveness, but in the scene's equivalent here, he plans on punishing Neji for his defeat. It has also recently been revealed that Hiashi was most likely behind the deaths of Kurenai's old team, because one member developed the Byakugan and refused to join the branch family- which would mean becoming a slave with his life in the main family's hands.
Cori Falls's fanfiction loves this trope. Jessiebelle, James's fiance, is portrayed as a monster who would murder Jessie in cold blood just to be able to keep James chained to her forever. In a non-shippy sense, Ash Ketchum is made into a holy terror whose only purpose in life is to ruin the trio's.
On the Poke Shipping front, Just Friends? transforms cocky rival Gary into an unrepentant douchebag who goes out of his way to crush Misty's spirit with cruel words.
Darkest Night twists Grimsley into a sadist who regularly rapes female challengers to the Pokemon League and has done the same to Shauntal. In fact, Grimsley gets this treatment fairly often due to his sinister appearance and having a habit of gambling.
Another fic by the same author, Silver Bindings, warps Well-Intentioned Extremist N into a psychotic, possessive Yandere who kidnaps Hilda and forces her to be his queen. When she protests, he has her thrown in a dungeon and beaten. When she won't have sex with him to produce an heir, he rapes her. When she escapes, he stalks her and tries to have her "soul mate" Hilbert murdered.
In Gym Leaders On Survivor, Morty is turned into a selfish, bossy, and overbearing prick who plans to cheat in the Survivor game so he can win.
Three's A Crowd is an interesting case, because the author was not intending to make any character Die for Our Ship. Despite this, when it comes to the competition between Ash and Gary for Misty's love, Ash inevitably comes off as the worse person, i.e., as somebody with the maturity level of a ten-year-old even though he is supposed to have aged between the first season and the time the fic is set, makes sexist comments and is so obsessive that he should probably never date any woman ever, lest he become abusive.
Devoted Obsession twists Keith into a psychotic Yandere who tries to murder Allura for falling in love with Lotor. By the same author, Infinite Jest portrays him as a thoughtless lout and ends with Allura desiring to kill him for being lousy in bed.
There's Something About Birdstyle warps Jun the Swan, canonically a caring Team Mom with a crush on Ken who shows a fiery temper sometimes into a crazed shrieking harpy who would gladly see Joe the Condor dead for various reasons, the two biggest being his comments about the length of her skirt (which were completely out of line and out of character for him) and for flirting with Ken, whom she wants to keep all for herself.
In an example where the character is definitely still an antagonist but still fits this trope, Ashura-oh in Tsubasa Reservoir Chronicle often has his negative traits exaggerated and is shown as abusive (sometimes sexually) to his adopted son Fai, when in fact in canon it was the opposite and Ashura-oh was the best parental figure Fai ever had, which made him going off the deep end and beginning to slaughter his countrymen due to the price of his magic and forcing Fai to put him into a magical sleep (in lieu of killing him, what Ashura-oh wanted) that much more painful for him. Both Word of God and Kurogane who is the one who eventually does put Ashura-oh down acknowledge that despite his later actions Ashura-oh's love for Fai was genuine.
The Amazing Spiderman 2, a fan sequel to thefilm, turns Gwen Stacy into a bitter mess who blames Peter for her father's death which results in, after they got back together, her insulting and berating him until they break up—to make way for the author's version of Mary Jane.
The Avengers' very active shipping fandom has given us these:
Motivations, despite not even being a canon character in the films yet, gives this involving Mockingbird. Depicted as Clint's ex-girlfriend who was reportedly very horrible to him, she gets seduced by Black Widow who tricks her into a filmed BDSM session where she's the submissive, after which she breaks up with her and uploads the video on the internet, which the Avengers, save for Clint, all find hilarious.
In Train, Mockingbird, having broken up with him twice before, gets back together with Clint but tries to force him to break off his connection to Widow, resulting in them breaking up again, and later revealing her to be a villain.
In Scar The Unsung Hero, Scar was abused by his brother, Mufasa, who told him that he was worthless, would gladly kill him, and gave him the scar, and was bullied by most of the lionesses plus Rafiki attempts to kill him to get revenge on Scar and Zazu gladly took part in bullying Scar with Mufasa and didn't bother telling the lionesses how Mufasa had treated him.
"Guerre et Paix" turns Steve, Will's father, into an unrecognizable drinker and Domestic Abuser, who, given that the story pairs Will with Warren, also gets the obligatory token homophobe role. It's also revealed that Josie cheated on him with someone heavily implied to be Wolverine, which is portrayed as a good thing. By the end, Steve's been carted off to rehab and Josie runs into the arms of her Hopeless Suitor, Mr. Boy. In the film, his worst trait is simply being a little too obsessed with the family name.
Trixie Belden fanfic You Have Got to Be Kidding Me! combines this trope with a health dose of Die for Our Ship, Slutshaming, and the Madonna-Whore Complex. Dot Murray, a character who appeared in only a minimal role in a single book in the series and had no dialogue at all, is derailed into a an unrecognizable ditz who is "fake", antagonistic, shallow, and interested in Anything That Moves. Never mind that Dot was canonically portrayed as likable and talented, Trixie herself admitted Dot was amiable and her jealously of Dot was her own personal fault, and Jim and Dot expressed a mutual romantic interest in each other. Here, none of the Bob-Whites like Dot at all, Dot is jealous of Trixie and rude to her, fixated on scoring Jim as a boyfriend, and continuously portrayed as "easy" and "loose" (ie, wearing a bikini to swim while the other girls wear one-pieces) in an attempt to villanize her.
The author of Moon Daughter, a Percy Jackson fanfiction, gives Annabeth this treatment. She is described as a "blond slut" who moves to the Aphrodite cabin, has "herpees from smoking Pot", and dumps Percy so he can "abbandon the prep lifestyle."
One particularly bad and extremely unsettling NSFW Degrassi fic called The Junkie Journals turns RileyStavros into a Mind Rapist, gay basher, drug addict, Jerk Jock, Villain Sue, and Karma Houdini. People walk away from the story feeling like they've come across the seedy underbelly of the world. All of this without warning or explanation. The main character of the story is also rescued by Peter, but it's a mystery why the so-called "author" couldn't just make up an OC who's evil instead of having to defile Riley's character.
Allecto's controversial critiques of Firefly are something of an extreme example of this; the actions of several of the characters (especially the Caucasian males) are often twisted in order to fit an extreme radical feminist reading or her preconceived notions about gender relationships, thus unfairly painting them as evil. One such example is Allecto's citing of Zoe calling Mal "sir" (presented as an ingrained habit developed from their days together in the military, simply for the reason that he was her superior officer and you call your superior officers "sir" in the military) as an example of Mal being evil and sexist. It's also fair to suggest that Allecto tends to let her preconceptions affect her views of the characters, such as her assumption that Wash must be abusive toward Zoe because every relationship between a white man and a woman of color that Allecto has seen has been abusive according to her.
The Friends fic Somebody Save Me (found here) has portrayed Richard - a perfectly decent guy whose relationship with Monica only broke up because of May-December Romance problems - as a drunken, selfish, physically and emotionally abusive asshole who doesn't care for Monica at all, so that Chandler can have an even better reason to be there for her. Why an OC abusive boyfriend cannot be made up, seeing as Monica dates dozens of men throughout the run of Friends, is a mystery. It's not the only one of its kind either. And it's not even like he ever got in the way of the Chandler and Monica pairing either.
The writers of 7th Heaven themselves do this to Mary in later seasons, turning her into the black sheep and disappointment of the family just because Jessica Biel posed topless in "Gear" magazine, infuriating the producers. It got to the point where her mother cringed at the mention of her name.
There's an entire website dedicated to doing this to Gwen Cooper of Torchwood. In them, she's reduced to a shrill harpy who wants nothing more than to get into Jack's pants and hurt Ianto in the process, no matter how little sense it makes at the time. There's even one story where the rest of the team can't stand her and murder her with no remorse. It makes one wonder why they didn't just fire her. For clarity's sake: the team has shown no sign of despising her in Canon, and she gets along well with Ianto despite the fact that she has two-sided UST with Jack (with whom Ianto is eventually an Official Couple).
Shenny authors in the The Big Bang Theory tend to ignore Sheldon's canonical averison to romance, his unique brand of love for canonical girlfriend Amy, his close, almost father/son like relationship with his roommate, Leonard, and the fact that while he does count her as one of his closest friends, his relationship with Penny defaults to Vitriolic Best Buds. Instead, Sheldon becomes a poor soul that's been in love with Penny for years, but that "asshole Leonard" (canonically an incredibly nice, often stepped over, guy) saw her first (ignoring the fact they are canonically a loving couple). Eventually Penny (canonically an independent weapons expert/sharp shooter/Girly Bruiser) will escape from under his abusive, whiny thumb, and will somehow find her way to Sheldon. One of the more egregious examples can be found here. While it does play with the typical amount of time it takes for Penny and Sheldon to find each other, it makes up for it by being written specifically to piss off fans of the canonical characterizations and also giving the Draco in Leather Pants treatment to Leonard's mother, Beverly, who in canon was an Abusive Parent that basically thought mind raping her kids was okay if she could use the experience to write parenting books and push them to the tops of their fields as adults.
Jealousy, a Chrom/boy!Robin fic that makes no bones about how the author feels about Sumia and her relationship with Chrom. Sumia goes from a sweet, clumsy, friendly girl who may have a crush on her prince to a whiny, petty little bitch who actively tries to break up Chrom and the male Robin's "true love". Boy!Robin doesn't come off too well either, turning him from a loyal friend into a just as petty jackass who tells Sumia "Well I had his child, so you can just back the fuck off."
SMW YEAHHH has him as the true villain and final boss
Brutal Mario looks to be the same, as implied by this early final overworld map. Note him riding in the clown car and the giant neon letters on the castle. Also, it does the same to Peach (who's shown to be working with Luigi, and can be seen as an unfinished boss in game) and Mario (who's apparently trying to take back the kingdom he ran as a dictatorship beforehand).
A Dark Fic author by the name of Stripe Pattern believes Byakuren, one of the unambiguously good characters in Touhou, is a psychotic monster in the doujin Love and Peace who genocides the entire human village, as she declares humans to be weak-willed, compared to the "strong" youkai, citing Buddhism to support her actions. The author actually quotes Byakuren's background to support the "fact" she's evil.
The same author also depicts Sanae, a happy-go-lucky ditz with racist tendencies and an overt fondness for youkai exterminationnote In canon, it's kept ambiguous as to whether or not "youkai extermination", as it's called by Reimu, actually results in the death of the youkai, though youkai characters supposedly exterminated pop up to complain about getting beaten up by the heroines, and the spellcard system was specifically instated to avoid actual death., as a very serious, sadistic Blood Knight who hates her own goddesses (people she loves in canon) who goes insane frequently.
One author, Zounose, has Byakuren come up with a way to keep the youkai she takes care of from eating people: use the corpses of recently dead babies to fertilize the fruit trees she grows, so that the resulting fruit technically counts as human fleshnote According to the local 'expert' on everything in the series' setting, and the youkai themselves (who are also prone to saying things to scare off humans), youkai require human flesh to survive. In fanon, this often ranges from being outright ignored or questioned, to proof that youkai are all sadistic eldritch abominations with an evil morality, out to murder humans. Just so we're clear that the author doesn't consider this to be a good thing, everything is done in sinister tones and Byakuren chews on the flesh of one of the fruits, signifying she's a technical cannibal, which is, for obvious reasons, a major no-no in Japan. Or anywhere else. The same artist also considers Sakuya to be willing to murder her own mistress, someone she's undyingly loyal to in canon.
There's a mildly popular series among a certain subset of fans revolving around Koishi, who can manipulate subconsciousnesses, being a cannibalistic, sadistic psychopath who drives all of Gensokyo into mass murder and torture. Note that, in canon, Koishi is considered to be brain-dead, with absolutely no thoughts or personality of her own.note This is, of course, only speculated by the resident Unreliable Narrator. In addition, Koishi's power to manipulate subconsciousness often gets warped into an evil power by cynical artists who consider the subconsciousness a very nasty place..
This is subverted in the doujin The Silence of the Rabbits, which depicts Eirin as a sadistic monster who does cruel experiments and is also a cannibal. The subversion is that this isn't the real Eirin, but a clone that Eirin created to manage the other clones that she created, but who went evil, incapacitated and imprisoned her. The clone wanted to come up with a poison that could kill a Hourai immortal, but despite her advances, she feared the original, and it turns out during the climactic battle between the Eirins that Eirin created her using the template for the Reisen clones rather than her own.
Bro Strider, Dave's badass older brother in Homestuck, is subjected to this in Out of His Depth, a Dave/Tavros fic, where he is both abusive and a homophobic jerkass whose only real purpose in the fic is to provide a reason for Dave to be reluctant to admit his feelings and someone for Tavros to stand up to. This is a bit Hilarious in Hindsight as the fic was written before the Post-Scratch Universe versions of the kids' Guardians were introduced and Bro's own counterpart Dirk is revealed to be both a pretty nice guy and, more importantly, gay.
Homestuck High depicts Sollux Captor as an actual demon who lives in a castle decorated with aborted foetuses and is attempting to forcibly impregnate Rose and Jade for some reason.
Mai in the fan comic How I Became Yours. Because how dare the woman be angry that her husband cheated on her and had a kid with another woman, clearly she deserves to be killed. Admittedly, she poisons Katara, causing her to miscarry, and later tries to kill her personally, almost killing the Out of Character but Heel Face Turned Azula in the process, but she comes off as more reasonable and sympathetic than the author intended her to be, which is surprising considering how much the author tried to derail and demonize her.
Aang in this Toph/Katara fic, Do you hate me?, is twisted into a swearing, angry jackass who throws rocks and hurts Toph on purpose. Granted, this does take place during "The Desert" and he was angry at Toph for not saving Appa, but he'd never go out of his way to hurt her. He, in fact, became upset after intentionally hurting buzzard-wasp hybrid creatures.
Embers. Katara's season three rage over Zuko getting Aang mostly-killed after she'd started to trust him is transposed into a season one homicidal fury that Zuko is of the Fire Nation and has the nerve to be a better healer than she is. Since then, Katara has been publicly humiliated by the amazing Zuko in three or four different ways and has been reclaimed as an acceptable being, but is being used as a prop in the new goal of showing that Aang is a horrible, selfish boy who is deeply intolerant of everything that doesn't fit into his childhood culture (which by the way is secretly evil).
Not only does Abuse Cycle (found here) take the Mako/Korra relationship down the road of Domestic Abuseand murder, but the author's notes make it clear that they think the signs of it on Mako's part are self evident in the show itself.
Voices (found here) does a bizarre inversion. The narrator (Korra's noncanon adopted sister) quickly falls for and latches onto Mako, while Korra is portrayed as being an irresponsible ditz who has no sense of manners or responsibility and needs her sister to be her "manager", because she can't take her Avatar training seriously (ignoring how, in canon, Korra loved being the Avatar more than anything). Given that the author proclaimed a rather loud hatred of Asami, in the notes of another fic, it's looking very likely that she's going to be getting the same treatment.
Pinkie Pie: Typically an insanely happy-go-lucky Genki Girl, the infamous fanfic Cupcakes derails her into a psychopathic killer who goes by her full name, Pinkamena Diane Pie.
Rainbow Factory does this to Rainbow Dash, who in canon is a brave, loyal Jerk with a Heart of Gold who would never abandon her friends. Rainbow Factory!Dash is an Ax-Crazy monster who not only oversees the deaths of failed fliers, but callously murders Scootaloo.
Frigid Winds And Burning Hearts has Celestia banishing Luna to the Moon in order to protect her rule over Equestria and then spending a millennium cementing her control over ponykind. What makes this a particularly egregious case is that the author claims to be a fan of Celestia and that he intended the story to be a Gray and Gray Morality tale.
Twilight Sparkle: I used to have this little toy named Mrs Buzzy. She was a stuffed bumblebee. And then one day while I was having a tea party, Celestia came, and she pulled out all of Mrs Buzzy's stuffing, and she filled her full of sliced bread, and she said, "There. Mrs Buzzy isn't real!" And then she told me to write a 300-page essay explaining what that proved. And I didn't KNOW! *cries*
Legends of Equestria blames the Everfree Forest on Celestia and has her establishing a dictatorship as soon as the world is free of outside threats.
The "Prayer Ponies" installment of The Prayer Warriors. While in this story, like the others it's easier to count the number of characters who aren't completely and utterly out of character, Celestia stands out, turning into a demon and exiling Charity Sue and Twilight Sparkle to a forest full of demonic trees just for mentioning Jesus in her presence, and appears to be the Big Bad of the story.
Jade Ring's "Apple Family Secrets" paints her as (underneath a mask of purity) shockingly arrogant and racist, blaming earth ponies for "saddl[ing] the rest of us with your feeble moral code" (i.e. monogamy) and addressing Applejack as a "bumpkin"- despite having just learned that AJ is her illegitimate great-granddaughter. She even briefly considers turning AJ into stone (and it's stated she had done the same to "so many would be radicals in the past") after AJ implicitly threatens to blackmail her with the knowledge of her affair- which verges on this trope for AJ as well!
Friendship is Witchcraft does this to Fluttershy, the leader of an evil cult to summon the god of destruction Smooze, and Twilight Sparkle, who kills Cadence and some classmates because she thought they were competition. Applejack, meanwhile, is a war criminal.
Teen Titans: Return of a Titan by RainthelingeringSentiment shows Robin to be a Manipulative Bastard who threw Beast Boy out of the team, afraid Starfire is attracted to him. In the end of the fic, he ends up as Slade's apprentice and Star ends up with Red X.
Geoff in the Total Drama fan fic Total Drama Luxury Tour. About a third of the way into the story, he goes from being one of the nicer competetors (as he is in canon) to being insane and overly violent, with one of his actions even giving Trent a serious injury that forces him to be removed from the game. After his elimination, he is shown to become so crazy that all he can say is "Bridgette," the name of his ex-girlfriend.
Transformers fan Raksha has a whole series of essays detailing how the Decepticons are the truly oppressed minority and actually noble creatures (yes, even Starscream and Megatron) and that the Autobots are the bad guys.
Not Just A Sleepover (found here) states right at the beginning that it's for non-Jean and Cyclops fans, and just gets worse from there.
Tsunami (found here) depicts Jean Grey as a cliché Alpha Bitch, with everyone vocally denouncing her as a two-faced bitch, all while her powers are degraded to Pre-Claramont Comics!Jean level in order to "fix" how "perfect" she is.
In Diamonds Are A Fuzzy Dudes Best Friend (here) Cyclops and Jean get this treatment. Jean gets pretty much nothing but abuse from Emma Frost with everyone stating how she 'deserves' it, while Cyclops is depicted as a moron who just follows either Jean or Xavier's every word.
Logan Vs Lance (here) has Avalanche portrayed as a cheating, abusive, Bastard Boyfriend to motivate the X-Men to beat him up so Kitty would have a reason to break up with him.