Joker isn't defined only by his brutality. he's defined by the fact that he CAN be that brutal, but his particular brand of insanity made it so that you didn't know wether he would kill you, or show you a card trick. He is entirely capable of blowing up a hospital, cancer ridden orphans and all, but only if the dice in his head roll that way on that particular day. Of course, then there's the argument that it could have been a parody of high school dramas, and gotten all the violence in there anyways. Cartoons ain't always for kids. It wouldn't have to even stick to the current canon Batman anyways. I mean, EVERYBODY knows about the Joker, but only the people who've stuck to Batman comics for a long time would that Joker was originally a thief known as the red hood.