In all fairness, Batman was kind of asking for it...
- "How dare you! I know evil is bad, but come on! Eating kittens is just plain... plain wrong, and no one should do it! EVER!"
- ―The Tick
These guys suck. They're jerks. You would never want to hang out with them. But the Heroes need people to fight, otherwise comics would have no conflict and nobody would read them. Where would Batman be without the Joker? Superman without his Lex Luthor? Luke Skywalker without his George Lucas? The Mayor of Townsville without his jar of pickles? Villains are evil, and they break the law, or at least commonly established notions of morality. Don't give them any candy, that would be very bad.
Do not trust them. Do not give them the keys to your grandmother, they will sell her when your back is turned. Also, they make terrible roommates. They don't even wash their hands when they're coming out of the bathroom. Did we mention that they eat kittens? Because they do.
Sometimes, when they get tired of doing their job, they might go to villain rehab. A hero that turns into a villain has been to hero rehab.