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:
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.
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.
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.
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 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.
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.
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.
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.
Speaking of Tsukasa, Ahoge renders her insane enough to believe that Konata's hair could be grown into some kind of plant and chop her head off with an ax for such a purpose. Can you say "literally Ax Crazy"?
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.
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.
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.
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.
"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.
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.
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.
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 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.
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, 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. Another is 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.
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 an Official Couple).
The Chinese Touhou doujin Yuyuko's Yukkuri Farm goes to the extreme and has Yuyuko as a literal baby eater (warning: link is to Danbooru and has NSFW ads). The author is extremely fond of depicting Yuyuko as a sadist and has a major dislike of her.
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.
Another 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..
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).
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.
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.
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.
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 Equestria and has her establishing a dictatorship as soon as the world is free of outside threats.
As you can see, poor Princess Celestia is really prone to this. Most of her demonized incarnations can be classified into three categories: Tyrant Celestia (who is often also a Villain with Good Publicity), Trollestia (sort of like Tyrant Celestia, only completely insane and usually darkly humorous), and Molestia (the less said about this one, the better).
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 perpetrated all sorts of horrible things in the war.
Rainbow Double Dashs Lunaverse does this to all the Mane Six by amplifying their flaws and removing their likable traits. This has become a contentious point even for the writers of the universe, as RainbowDoubleDash has said that his original intent was only to (anti-)villainize Twilight.
YMMV on that, as the initial impressions were from the point of view of Trixie, and subsequent stories have expanded on their motivations to make them more understandable.
Twilight Sparkle also has her own "Cupcakes": The Experiments of Twilight Sparkle, in which she tortures her friends under a very vague pretense of science, but more likely just for the kicks of it.
Pony Dot Mov does this to just about all of the Mane Six. Applejack (this version of her is called Jappleack) is a violent southern hick who often abuses Applebloom, Pinkie Pie is a drug-addled attention whore, Rainbow Dash is even more of a Jerk Ass than usual, Fluttershy is Ax Crazy, Rarity enslaves Mexicans, and Twilight Sparkle is an incompetent Mad Scientist. Certain of them do get some Character Development, the "Ask Jappleack" Tumblr page comes to mind.
Completely and Utterly Averted in Life In Manehattan . The Author of the original story has gone on the record that this is Not gonna hit the rest of the canon mane six (Twilight Still being the Element of Magic). To the point that Rainbow Dash (who is the author's least favorite member of the canon mane six) is not showing up at all just to avoid this trope.
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.
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.
Geoff in the Total Drama Island 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.
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, potraying him as Axe CrazyJerk Ass who gets rightfully deserved punishment.