Villains are murderers. Psychopaths who want to see the world burn and will do no matter what it takes to make sure that it does.
It's different than the general 'bad guy', who walks a crooked line but doesn't much like the idea of the world ending or death. What superheros and those people who act all superior to those they try and bring down don't know is that there's all sorts of different levels on the types of bad and evil that we've got out there. They see that it's not good and that's all they want to see.