We've talked about broken rules and in fact the film itself seems to be all about what happens when you start breaking them. Let's talk about the one rule that doesn't get broken, and what it means when the bat and the clown lock horns.

That one rule is Batman's, the only one he rigidly adheres to: no killing. Seems easy enough to follow until you realize what kind of pressure it causes in his line of work and how much easier it was all be if he broke it just once. It's also a pretty heroic rule, and Batman needs it to separate himself from the people he's fighting. (Nolan made that very clear in the first film, when Bruce Wayne's refusal to kill created the "let's burn each other's house down!" rift between him and his mentor.) Having defined his morality by that rule, Batman really, truly doesn't intend to break it. And since the Joker basically wants to show everyone that you can't live by rules, he's going to have to get the Caped Crusader to break it… even if that means goading old Bats into killing him.

(Batman never quite breaks it of course, though he eventually makes everyone think he did… but we'll get to that in the "What's Up with the Ending?" section.)

The rule thus becomes the essence of Batman's struggle with the Joker. If Batman sticks to it, he shows the Joker that people can adhere to standards, and that anarchy isn't the only logical way to live. It focuses their larger conflict into a simple matter of principles. Either Batman stands by them, or he doesn't. Everything else is academic. (Well okay, there's the whole question of whether the city's going to descend into complete insanity, but what are ya gonna do?)