Heroes have an Evil 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, their desire for revenge, a failed love relationship, betrayal from the good side to the dark side, or any other unresolved traumatic experience in which they question whether or not the good side is worth fighting for.

A "Hero turned to the Dark Side" (Fallen Hero for short) is usually the end-result of a protagonist exposed to a Corrupting Influence, Brainwasher, and occasionally an Extortionist, Blackmailer, or even a Bond Destroyer.

In some situations, some heroes turned to the dark side can become Complete Monsters to add drama as they are willingly to be evil without remorse, redemption or positive qualities after all.

But at the end, the heroes will learn their mistake and go back to be heroes like they always have for years or realize that darkness is hurting/pushing people away and get rid of this madness inside of him/her.