Why would a societies hero become the new villain?

Others have already said great ideas here, but I thought I'd throw in my 2 cents as well, being a big fan of superheroes. Perhaps the hero originally followed the superhero's code(not killing the bad guys, no collateral damage etc.) but was later getting critisized for that, as his ethical ways didn't prevent villains from returning or provide sufficient deterrent for others. So he changed his ways, became more violent, more authoritarian and took peoples advice too far. In this scenario, he still considers himself a hero and believes he is doing good using ruthless methods. The harder question is, why the villain would turn into a hero. Perhaps he simply enjoys the possibility of fighting against the hero, only this time with the whole society supporting him. Perhaps he realizes that the easiest way to achieve fame, money and power is the way of the hero rather than that of the villain. In this scenario the characters intentions never change; the hero changes his methods, but still believes in a form of justice and the villain is just as self-centered as before, but now achieves his goals through publicly accepted ways.

 

There's the part where a villain pushes the villain too hard and then causes him to snap like with Superman.

 

You're welcome!

But, I want you to realize that if you had already written all that in, then you already knew what sorts of things that you had to do in order to mold the storyline. You just may not have known why you knew what was logically necessary and "felt right" to write. So, now you do! Good job.