Probably, I mean, he killed for what in all intensive purposes was my son.
I just watched "Under the Red Hood" though, and they explained it in a good way.
Batman isn't really like you or me. Rational people who could say... kill Hitler or the Joker and be fine.
Batman is a guy who's parents were killed by muggers... and his response was to become this grim, emotitonless guy who's only duty in life is to fight crime, who trained martial arts forever, because some random mugger killed his parents.
It's not that he doesn't kill because it's immoral. He doesn't kill because he's not well... and if he did start to kill, it'd be tough for him to stop. It's not like he started crimefighting BECAUSE of the joker. He started crime fighting because of some guy who shot his parents. Some guy Batman would very much like to kill.
Some guy just like most minor criminals.
The difference between us and Batman is... we're sane.