that are actually villains
Jaden from Yu-Gi-Oh went to a demonic world for saving his friends but it was a trap and he got possessed which turned him into an Evil Overlord.
Shiro from Deadman Wonderland began as an innocent character, but years of torture developed a murderous alternate personality inside her known as Wretched Egg.
Irene started as a good person but harsh neglects changed her fundamentally especially because her husband can not accept her after her Dragonification.
Lelouch from Code Geass pledged to free Japan and create a better world for everyone. However, his moral compass shifts and he becomes worse than the evil empire he conquers.
Griffith from Berserk begins as a team mate of Guts, but as the story progresses Griffith turn from an ally to a murderous tyrant who is hungry for land and power.
Tokyo Ghoul's story revolves around Kaneki Ken who became a Ghoul-Human Hybrid, which is the solo thing that he hated the most throughout the series.
The self proclaimed God of Justice from Death Note himself became the criminal which he hated in the start of the anime series.
Attack on Titan is a masterpiece. The story telling is amazing. The nerve chilling transformation of Eren Yeager is something that cannot be explained by words.