I was aware that some readers of the book objected to the Batman sharing a laugh with the Joker at the end. But the joke itself and the laughter is a switch from the gritty realism of the Batman offering to rehabilitate the Joker, poised on the edge of knowing that they were inevitably going to kill each other otherwise, to symbolism. The joke is a symbol, the laughter is a symbol, and the playing card on the back jacket is a symbol: that, at tne end of the day, the two characters have manifested their grief in fundamentally similar ways (whereas Gordon is stronger than them both).
... nah. That's a very fucking long bow. Batman reaches out and grabs the Joker by the shoulders while they're laughing (unless he's snapping the Joker's collarbone). His hands are nowhere near his neck.
Plus, it doesn't flow through from the story - there's discussion about redemption, rejected because the Joker says (in the context of the joke) that he doesn't trust a crazy man (Batman) to help him escape his own craziness.
And, as if Batman is going to kill the Joker in front of a dozen police cars.
The rain at the end is how the book begins. At best, the rain splattering into the water is either symbolic of grief, or symbolic of nothingness - the black drops hitting the black water. Its the lack of meaning in the world which has driven the Joker to his insanity. The laughter stopping doesn't mean the Joker is dead. It means that once the Joker is hauled away to jail, once Batman goes back to sit to brood in his cave, once the laughter and the action stops: all that is left is nothingness, the lack of meaning to life, the lack of reasons behind the hideous deaths which have scarred these two men. The cause of the Joker's insanity, and Batman's insanity, is that there is no reason, purpose or meaning behind the events which have caused them to get to where they are.
