Due to an ignored warning about corruptive curse within the runeblade Frostmourne, Arthas Menethil, upon touching it, inevitably succumbed by its curse and became a cold and ruthless death knight he become today.
Chirin no longer the cute, innocent and harmless lamb he once was due to result of his rigid training by Wolf not only made him evil and forgot his original motivation and his loved ones, but he also physically warped into abomination.
Spongebob becoming aggressive (which due to his naive persona mistaking the behavior as simply become cocky), forgets to be nice by Plankton's deception though this does not last long, as his redemption wind up undo the villain's scheme to turned the beach into Mega Bucket.
Terra's own decision to seek Slade's aid in controlling her powers turned out costs her friendship with Titans. Slade's influence to her is not sole reason for her fall from grace, as the matter of fact, Terra's own naive and vulnerable personality are what made her wind up choose to seek Slade's aid in the first place and the criminal mastermind merely did the rest.
Cassie Cage and Jacqui Biggs enslaved by Skarlet's Blood Code.
April O'Neil and Casey Jones under vampirism of Count Dracula.
Raphael turned by Dracula into a vampiric servant.
Anakin Skywalker succumbing to the Dark Side due to Palpatine's influence that drove him to turned against Padme and brought downfall to Jedi Order. Worse, his said action left the everlasting impact that greater than Luke expected as this inspired Kylo Ren to become evil like Anakin too.
The Iron Giant, angered over Hogarth being supposedly killed, retaliates against the US Army while forgetting how dangerous his weapons are.
Bumblebee and Prowl are turned into zombies by space barnacles.
Walter turned into a hunchbacked ogre by a Haunted Mask.
Sprx about to be corrupted by the Fire of Hate which started to overcome his will.
As result of murdering Vortigern with her magic, Nimue has allowed herself to be corrupted by Darkness, resulting her transformation into Dark One.
Shock Rock under the influence of the High Override.
Eduardo Rivera turned into a Ghost Clown after being bitten by an Evil Clown Toy.
Superman becomes the villain after he was manipulated by the Joker into killing his wife, Lois and destroy Metropolis, though he was once a pure hearted hero.
Dark Danny was created as a result of the ghost forms of Danny Phantom and Vlad Plasmius fusing together after Danny lost his family and friends.
After rejecting Kyle and being manipulated by Cartman into staying with him and eating junk food, Heidi turns fat and becomes a version of Cartman.
Wasp has become a mentally broken revenge-crazed 'bot after spending 50 years in Autobot custody after being framed for being a Decepticon double agent by Bumblebee, who was manipulated by Longarm Prime to prevent the Autobots from finding out that Longarm was the real double agent Shockwave.
Kylie Griffin possessed by Achira.
Carly Beth is being possessed by the original Haunted Mask.
Robin mind-controlled by Slade.
The Black Prom caused by Carrie White's rage after being dumped in pigs blood and being tormented for her whole life.
Natsu in his demonic form, E.N.D.
Lawrence Fletcher turns evil after he was shot by Make-Everything-Evil-Izer because of Dr.Doofenshmirtz and Rodney.