Heroes turned into the Dark Side

Many heroes walk a thin line between good and evil, indeed many heroes have fallen into despair, rage or even madness and once proud heroes descend further into the path of evil, often fighting against people they once protected or fought alongside.

Originally protagonists or somewhat good guys, these villains turn evil due to events in their lives such as being corrupted by another villain, the desire for revenge, a failed love relationship or even betrayal from the good side to the dark side.