The interesting about the Joker is, while he is perfectly aware that he's a homicidal maniac, HE ENJOYS IT. That's what makes him the Joker - he sees the world as a sick, twisted, meaningless place, a point of view that would drive anyone else to despair, but he finds it funny, and he can't really understand why no one else does. So, in a sense, him destroying the universe is one of the sanest things he's ever done - he's finally ACKNOWLEDGING the fact that no one else is getting the joke, and the reason is that it's NOT A JOKE, and if it's not a joke, then his whole career has been pointless - there's no punch line, as it were - and if there's no punch line, then why should the world exist at all? If HE'S wrong, then so is the universe that spawned him, and it's better off destroyed, so a new one can arise from the ashes. It's still crazy, but it has a certain melancholy logic to it.