Ron the Death Eater/Western Animation

Examples of in  fan works include:

Avatar: The Last Airbender

 * Infamous example: 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.
 * 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 creatures that are a combination of buzzards and wasps.
 * 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).

My Little Pony: Friendship Is Magic

 * Pinkie Pie. Typically insanely happy-go-lucky, the infamous fanfic Cupcakes caused a good chunk of the fan base to derail 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 Complete Monster who not only oversees the deaths of failed fliers, but callously murders Scootaloo.
 * Pattycakes inflicts this on Fluttershy, turning her from a Friend to All Living Things who is able to resist Discord's Hannibal Lecture through Incorruptible Pure Pureness into a creepy "age play" fetishist who brainwashes Rainbow Dash and her other friends into an infantile state.
 * A number of fics involving Princess Celestia, who in canon is a kind, laid-back, accessible Reasonable Authority Figure who loves her subjects and sometimes engages in lighthearted mischief. In fanfic, "Trollestia/Molestia" is a cruel and shameless prankster, a manipulative bitch, a tyrant, or worse; examples include:
 * Pony Psychology Series, at first.
 * Past Sins, at least until the revision, debatably.
 * In Severing and Reconnection, Princess Celestia tells Twilight to kill all her friends. Yeah.
 * Frigid Winds and Burning Hearts, wherein the entire history of Equestria is apparently a lie to keep Celestia in power.
 * In Friendship Is Magic Bitch, Celestia is an autocratic caligula
 * Miss Cheerilee in Cheerilee's Garden, in which the titular Cheerilee, a school teacher, becomes depressive and psychotic when her class of small children have the audacity to act like a class of small children. Her answer to this crime? Murder all of them in front of the town in the guise of a "play".

X-Men: Evolution

 * Not Just a Sleepover (found here) depicts Cyclops aka Scott Summers as a humorless, oppressing, and incompetent would-be dictator who just hates the Brotherhood for no reason at all.
 * 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 Dude's Best Friend (here) Scott and Jean get this treatment.
 * Logan vs Lance (here) has Lance 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.

Other works

 * In the Teen Titans fic Intertwining Hearts (found here), Robin behaves like a jerk when Starfire tells him she wants to include Raven in their relationship (normal for Tamaran). So instead, Starfire and Raven include Beast Boy in their relationship. Robin is not that evil, however (though he does briefly attempt breaking up Starfire and Raven), and the fic ends with him having an emotional breakdown.