How we cite our quotes: All quotations are from The Dark Knight.
Quote #7
BATMAN: I have one rule.
THE JOKER: Oh, then that's the rule you'll have to break to know the truth.
BATMAN: Which is?
THE JOKER: The only sensible way to live in this world is without rules.
Here, Batman and the Joker define the real terms for their conflict. The Batman won't kill anyone. That's it. And if the Joker can induce him to kill someone, even if it's the Joker himself, then Batman has perfectly proven the Joker's point. Easy on that trigger finger Bats; you're playing for keeps here.
Quote #8
THE JOKER: You know what I've noticed? Nobody panics if things go "according to plan…" even if that plan is horrifying. If tomorrow, I tell the press that, like, a gang-banger will get shot, or a truckload of soldiers will be blown up, nobody panics because it's all "part of the plan." But if I say that one little old mayor will die…well then everyone loses their minds!
For a crazy man, he kind of has a point here. Sometimes, the rules become more important than the good things they're supposed to make happen. Again, it's the "destroying the village in order to save it" thing. Sometimes we have to break the rules to do the right thing, and if we do, then how do we even know what the right thing is? It's a thorny question, though most of us prefer to contemplate it without burning hospitals down or driving public officials crazy.
Quote #9
THE JOKER: I think you and I are destined to do this forever.
The Joker implies that this is how it's going to be going forward, which is another way of saying "these are the new rules, that you and I have created through this little battle of wills." Even he can't escape the need for them sometimes.