The result of either 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, uses canonical evil actions as a justification as to why they are irredeemably evil even if the canon says otherwise, and has every possible negative trait of the character 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 through their canonical blunders or being an obstacle to the author'sOne True Pairing, especially if said character is a member of the Official Couple set. Or maybe they're getting in the way of the Mary Sue's spotlight.
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".
Unlike Draco in Leather Pants, this can also be intentionally done as a comedy trope, usually of the black variety. In this much rarer variant, the character in question is turned into The Jerk (or worse), but the other characters' relationships with them aren't changed much. That way the main characters are exposed to, and thus constantly made to suffer by, a villain and good times are had by the audience.
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 (or have it lost for him) 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), The Scapegoat (when a character is blamed for an event they're not responsible for), Historical Villain Upgrade, Dark Fic, Rooting for the Empire, 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, a Writing Around Trademarks version of 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.
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.
Virgil 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.
The Doctor in Voyager also writes a holo-novel (which is supposed to be about the abuse and objectification of holograms) featuring Expys of the ship and crew of Voyager, making then far more abusive and sleazy, and who ultimately have the self-insert protagonist Doctor shut down and reprogrammed. When the Voyager crew finds out, they're offended that he would paint them in such a bad light, eventually convincing him to make enough changes that the cast aren't just Voyager crew look-a-likes with mildly different names.
The writers of Seventh 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.
The Torchwood episode "Fragments" depicted the usually sympathetic Doctor WhoMen In Black organisation UNIT as a vindictive group of sadists who "disappear" people on little provocation and keep them in Hellhole Prisons forever. This was very unpopular with the fandom and widely assumed to be a case of this trope.
And then he did it to himself with "Birdman Kicked My Ass", wherein the title superhero beats the crap out of him for trespassing on his property, whereupon Willis comes back to Birdman's house in the middle of the night to throw a brick through his window.
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, to which Tina answers that without Angel, no one would have died.
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.
One episode of Arthur features Ladonna telling Fern about her bingo-playing aunt Mable and a mysterious bingo opponent. When Fern rewrites the story (without Ladonna's permission), she makes Aunt Mable the Big Bad by combining her with the bingo opponent (and changing her name to "Sable"). Ladonna calls Fern out on it.
In "Itchy & Scratchy & Marge", Roger Meyers Jr. and his lead writers portray Marge is a cartoonish squirrel, who continuously tells the cat and mouse duo "Don't do that!", while they hit each other with baseball bats, before whacking her head off.
In "Homer Badman" with the TV movie Homer S.: Portrait of an Ass-Grabber, Homer, played by Dennis Franz, is portrayed as a cackling, dog-kicking pervert.
Ashley: No, Mr. Simpson! A cat is a living creature! Homer: I don't care! [runs over cat] Now I'm gonna grab me some sweet! Ashley: No, Mr. Simpson! If you do, I'll scream so loud the whole country will hear! Homer: Heh, with a MAN in the White House?! Not likely! Muah ha. Muah haha. MUAH HA HA HAAAAAA!
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.
Digimon Adventure fans has a recurring habit of demonising Yamato/Matt by Tai(chi)/Sora shippers by either portraying him as an unfaithful man to Sora, an abuser who attempt to beat up Sora just because she hung up with Tai a bit more often, or someone who prioritises other activities over his girlfriend to the point that Sora feels more closure in hanging out with Tai than Matt. Hold on is a good example of fan fictions like these by demonising Matt to the point that he is willing to beat up Sora and even kill Tai with a gun.
Most of Gallantmon Of The Hazard's fictions tend to demonize Takeru/TK just because he is in the way of his sacred Daikari (Davis/Kari) pairing whom he treats Davis like some kind of Gary Stu. Digidestined of Minerva is a classic example as he directly turns TK into an evil criminal who does not hesitate to kidnap Kari into a sex slave.
It Falls on Me has Yamcha dedicated to ruining Bulma's family name, plot to steal her wealth, and kill Vegeta. This is contradictory to canon in several ways; Yamcha is shown remaining on good terms with her after their breakup, its stated in series that Yamcha is not interested in power or status, him, Yamcha entering the relationship which was initiated by Bulma before he knew of Bulma's wealth, and being one of the characters portrayed as more accepting of Vegeta.
The Legacy of Kaiser does this to Yamcha (and, inadvertently, Goku) by having him laugh over the memory of his friends dying.
Those Magical Three Years (once found here) does this same thing on Yamcha, turning him into an abusive drunk who apparently regularly cuts Bulma.
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's revealed that Maho actually loves Miho dearly, and serves as a Nishizumi heiress so that Miho will be free to choose her own path in life.
In Girls und Panzer The Eagles of Oarai, Shiho not only disowns Miho for not doing tankery the Nishizumi way, but also Does Not Like Men to an absurd extreme, hating their getting involved in tankery through fighter planes, and wanting to see them as "nothing more than slaves to female kind." After her school loses, and one of her pilots almost dies because her plane didn't have an ejection seat, she pulls a gun on the male lead and gets arrested.
An Alternate Keitaro Urashima isn't kind to Granny Hina or the Hinata Girls. Shinobu is the only tenant who doesn't have a crippling case of It's All About Me. No matter how severe the consequences of their actions, as well as constantly being informed that they're responsible for their own actions and have to deal with the fallout, they continue to blame everyone but themselves, recasting themselves as the victim.
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 PokeShipping 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.
In canon, Teru is an Aloof Big Sister who has grown cold to her younger sister, Saki around the time the series starts, to the point at which she refuses to speak with her or admit that she has a sister, although she does have more sympathetic, moments, such as when she appears completely shocked after one of her opponents collapses after their match. In Saki After Story, found here, she goes into an Unstoppable Rage and viciously attacks Saki for trying to talk to her after defeating her team in the tournament, and also attacks Nodoka when she intervenes, putting both in the hospital.
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.
I'm Here to Help does this to just about every canon protagonist. Luna is portrayed as more of a jerk than she was in canon, as well as being overly suspicious for not trusting Emerald with the identity of Serenity. Tuxedo Kamen/Endymion is described as a priss who ended up becoming king of Crystal Tokyo by dumping Rei and Sailor Moon (with the implication of him two-timing both girls) and marrying Serenity purely for ladder-climbing purposes. Even when Emerald learns that Sailor Moon and Serenity were the same person and thus Endymion hadn't been callously abandoning Sailor Moon, Endymion is still viewed in a negative light, complete with the thought that he let himself be brainwashed so he could try out being evil without the guilt, and that he was "sleeping [his] way to the top" with Beryl, like he apparently did with Serenity (never mind that Endymion was brainwashed into doing so). The future senshi, meanwhile, are depicted as violent, brutal Knight Templers who have no mercy for their enemies. Because this is all told from the point of view of Emerald, it's difficult to tell exactly how much of this is meant to be seen as accurate. Emerald himself is insane, and the author's note at the end says that the deliberate exclusion of Neo-Queen Serenity from the story was meant to keep things ambiguous. However one section of the story has the future Jupiter and Mars discussing one of Emerald's plans, which involved having two children aid him, and heavily imply that they hurt and/or killed said children and refuse to let themselves feel bad about it, indicating that the future senshi are supposed to be as bloodthirsty as Emerald says.
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 Mufasa 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.
Hilariously subverted in How Aubrey Got Arrested. Aubrey tries to host a Haunted House for a fundraiser, but it fails due to the laziness of the Bellas. This causes Aubrey to snap and start killing the other Bellas. However, this is later revealed to have all been an act.
Trixie Belden fanfic You Have Got to Be Kidding Me! combines this trope with a healthy 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 jealousy 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 (which you can find here), a Percy Jackson and the Olympians 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."
ThisDresden Files fic makes Marcone an obsessive Yandere who keeps a shrine to the object of his affections. Marcone may have been unrepentantly Pragmatic Evil in canon, but it's still greatly OOC for him to make a museum exhibit out of someone's hair!
Shenny authors in the The Big Bang Theory tend to ignore Sheldon's canonical aversion 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.
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 (despite the fact that she could "kill me [Wash] with her pinkie") 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.
And then there's SF Debris's reviews of the various Star Trek series. Picard of The Next Generation is generally portrayed as a fanatical child-hater who would happily order Worf to saw up a teddy bear in order to interrogate a child, and may or may not have ordered Jack Crusher to his death solely to increase his chances of getting with Beverly; Voyager reviews, meanwhile, react to the uneven characterization of Janeway by portraying her as a demented supervillain who views her crew as entertainment dispensers - even inventing a holiday, "Condescending Bitch Day", which happens every day and involves selecting someone to torment - and is planning to take over the entire Alpha Quadrant when she gets back. Chuck Sonnenberg has even joked that Janeway is his favourite villain.
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).
The author of the Buffy the Vampire Slayer fic Penny For Your... is especially blatant when it comes to clearing away Buffy's other boyfriends in order to leave room for her "one twoo wub", Spike. Angel is depicted as an arrogant idiot child with a god-complex, while Riley is turned into a sadistic monster whose attitude actually improves once he's turned into a vampire. And of course, Buffy's friends are all okay with her getting with Spike, even the vampire-hating Xander.
Spuffy authors really don't give Angel or Riley a break (to be fair, Riley's flat characterization in the show was kinda asking for it). The fanfic Five Words or Less is another, albeit much milder example. Riley is portrayed as a bastard boyfriend who likes to gleefully beat up Spike for no reason other than he's proving to be a better match for Buffy, and while Angel isn't bashed nearly as much as Riley, there is one instance where he comes off as a little too childish and petty.
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."
Mario is seen like this by great part of the fanbase, mostly because of Luigi (Luigi doesn't get as much screen time as him, but many people make it look like Mario mistreats and abuses his brother), the endless enemies he stomps (other playable characters aren't blamed), and his relationships with Daisy and Pauline (both relationships are dead now) besides Peach.
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).
Luigi: "Brother, I've become a wielder of Black Magic."
In the Super Mario World creepypasta I HATE YOU, the level that the storyteller discovers ends with Luigi facing off against Mario, apparantly sick of always being in his brother's shadow. He also seems to have been working with Bowser the entire time. What follows afterwards is a cutscene where the two fight, ending with Mario punching Luigi into a pit of lava in the arena.
A Dark Fic author by the name of Stripe Pattern believes Byakuren, a Buddhist magician in charge of a temple, is a psychotic monster. In Stripe Pattern's doujin Love and Peace, Byakuren 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.
In the RWBY fic Trials of a Workaholic, Weiss is depicted as sexually abusive. This was apparently based off her Squeeing over Pyrrha's battle skills for all of two scenes. Why the author even thought that it was sexually motivated, we don't know.
In Pinned, Pyrrha- ofallcharacters- is depicted as a rapist. This is the girl whose greatest canonical flaw is that she's so compassionate she can't speak up for herself.
Bro Strider, Dave's badass older brother, 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.
Troll FicHomestuck High depicts Sollux Captor as an actual demon who lives in a castle decorated with aborted fetuses 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. It's also incredibly out of character for Toph since the canon Toph wouldn't put up with that sort of treatment from him for anymore than a second.
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's 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'll be getting the same treatment.
The violent Troll FicSteven And Amethyst 2: The Choice Between Good And Evil, from the same author as the above stories that do this to Hiccup, portrays Connie as a psychotic, unclean, partial Asian stereotypenote The story describes her as Indian (she's Ambiguously Brown in canon, but confirmed by Word Of God at some time to be Indian), and she uses shuriken as weapons and has Baguazhang skills. Neither of those are canon. As far as we know. And it also describes certain points of her as "chakra". who's trying to break up Amethyst and Steven to have Steven for herself. This might have been a Stealth Parody of guessing that she would be subjected to Die for Our Ship, since this was published around Connie's debut and beforehand pairings like Steven/Amethyst and Steven/Pearl were popular.
It's hard to tell in The End of Ends, but the author was aiming for this portrayal of the Titans, even though Beast Boy's Wangst, stalking, descent into outright villainy, and eventual death are behavior commonly associated with characters who get this treatment... And this is his Creator's Pet we're talking about!
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.