Alex and Richard of the Anime Egotists talk about the Heroes who could've become Villains. 

Like in wrestling, some people have more fun as bad guys! These are the heroes who needed a darker shift in tone; to head to the dark side. Some permanently and some are just temporary, but regardless...This is Evil...EVERYTHING IS EVIL (Back when he was actually cool..)  

Let us know what heroes you'd want to go through a heel turn!