How we cite our quotes: All quotations are from The Dark Knight.
Quote #7
THE JOKER: I had a vision, of a world without Batman. The mob ground out a little profit and the police tried to shut them down, one block at a time. And it was so...boring.
Kinky…The Joker defines himself here not by his own actions, but by the fact that he needs Batman to be completed. His world is no longer worthwhile unless he has someone like Bats to challenge him: probably the closest a guy like him has to a relationship with another human being.
Quote #8
THE JOKER: Tonight you're all gonna be part of a social experiment. Through the magic of diesel fuel and ammonium nitrate, I'm ready right now to blow you all sky high. Anyone attempts to get off their boat, you all die. Each of you has a remote... to blow up the other boat. At midnight, I blow you all up. If, however, one of you presses the button, I'll let that boat live. So, who's it going to be: Harvey Dent's most wanted scumbag collection, or the sweet and innocent civilians? You choose. Oh, and you might want to decide quickly, because the people on the other boat might not be so noble.
This is a bet over who people are: are they essentially awful, as the Joker maintains, or ready to believe in good, as Batman's convinced they are? The whole story boils down to which side those people on the ferry choose, and by extension, who they decide to be.
Quote #9
BATMAN: The Joker cannot win. Gotham needs its true hero.
GORDON: No!
BATMAN: "You either die a hero, or you live long enough to see yourself become the villain." I can do those things, because I'm not a hero, not like Dent. I killed those people. That's what I can be.
GORDON: No, you can't! You're not!
BATMAN: I'm whatever Gotham needs me to be. Call it in.
Batman's identity is flexible, amorphous, and able to be twisted around like Silly Putty if Bruce Wayne wishes it so. In this case, he believes it would do the most good if Batman becomes a villain: taking Harvey's identity onto himself in order to preserve the good man that Harvey used to be.