Re: The Joker's background?
I wouldn't rule out the possibility of him being suicidal - I mean, he taunts Batman into running him over, he puts a gun in Dent's hand and forces it to his own forehead and he howls with glee when Batman throws him off Prewitt Building (and almost looks confused and disappointed when Batman catches him just in time). I think the Joker definitely likes the idea of his own death, but I would agree that for him it should come with a "higher" purpose - like making Batman or Dent kill him so they would be corrupted by the very act. I don't think he minds dying if that means that someone else would come in his place - look how happy he is when informing Batman about the "heroic things" Two-Face's up to. I believe that the kamikaze comparison isn't far from the truth behind his mindset.