Due to an ignored warning about the corruptible curse within the runeblade Frostmourne, Arthas Menethil, upon touching it, inevitably succumbed by its dark curse and became a cold and ruthless death knight he became to this day.
Because of adopting the lifestyle that mostly defy his nature as a sheep (living and training in the way of wolves) out of revenge, Chirin gradually forgot who and what he was that by the time he became more than capable to avenge his mother, he has grown into neither sheep nor wolf, but rather an abomination.
From the moment Terra decided to join Slade so she can properly control her powers, little did she realized she became as ruthless as her mentor through overwhelming her former friends. Even so, whatever remaining compassion she had, as well as Slade's betrayal, eventually forced her to see her errors and redeem herself.
Because of Palpatine's influence that worsened his issues and failure to save his wife Padme, Anakin Skywalker succumbed to the Dark Side and turned against Padme and brought downfall to Jedi Order, becoming Darth Vader in process. What made his case even tragic is that not only it left a lasting damage by helping the rise of the Galatic Empire, but his downfall was the testament of how hopelessly flawed Jedi Order had became during Anakin's days as a Jedi.
Ben Solo was burdened by his family legacy and corrupted by Palpatine (whispering in his mind through his creation, Snoke) since a very young age. After his mother sent him away and Luke attempted to murder him in his sleep, Ben felt unwanted by his family, and went to Snoke as he was the last person he trusted. He embraced the Dark Side as a way to find his true path, but by the time he assumed the identity of Kylo Ren, he was so consumed with hatred that he was a very different person, forgetting who he truly was.
After discovering that she is the daughter of Mother Gothel who abandoned her for Rapunzel, Cassandra became bitter and allowed herself to be corrupted by the Moonstone in her volition. As a result, she becomes progressively evil and vengeful.
As result of murdering Vortigern with her magic, Nimue has allowed herself to be corrupted by Darkness, resulting her transformation into one of its Dark Ones.
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.
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.
Because he was resurrected by his older brother Zeref as the ultimate Etherious, E.N.D. Natsu is doomed to suffer this fate, something that later confirmed when Lucy seemingly died on his hands. At that very point, he turned into his demon form and attacked anyone who stand between him and Zeref until is stopped by combined efforts of Erza and the recovered Lucy who used Book of Zeref, which started it all, to defeat the latter and restored Natsu back to himself.
Kaede/Lucy had the Diclonius DNA voice in her head trying to convince her to take revenge and kill people, as it wanted her to wipe out the humanity to replace them with the Diclonius race. After a series of misfortunes and betrayals, including the murder of her puppy by bullies and feeling betrayed by the guy she had fallen in love with (Kouta), she decides to surrender herself to the voice’s wishes as she now had developed hatred towards humanity. She becomes a cold-hearted serial killer, and all the qualities of her personality are obfuscated by her murderous desire.