Every good story needs a good old-fashioned villain. Bad guys are the reasons good guys look good, to begin with. But here's the thing about bad guys. There is always a reason why they turned the way they did. For some, it's money. For others, it may be revenge and the likes.
However, from where these villains stand, they are the victims of their story.
See, each of them had their own reasons to do what they did. Except the Joker. He did it for the lolzz!