My quick analysis of the movie. (spoilers obviously)
I believe that mediums that try to deliver a message do so in one of two ways, they either make a statement or ask a question. Dark Knight asks a question of it's audience. I divide this question into three parts. What is morality, what makes us moral, and what does it take for us to become immoral?
The question is asked by means of the Joker. The Joker is an agent of chaos, the movie demonizes him into being something less than human but more than mortal. All the characters in this new batman saga are very human. Unlike in the past where they were over the top, campy, supervillains and heros, these people are all real, made to be believable. The scarecrow didn't where a goofy costume, he had a burlap sack over his head. He was just a psychologist, an ordinary guy with a few hang ups. Batman is more human in this than he has been in any past movie or TV iteration. Harvey Dent is also very human. The joker on the otherhand is human, but at the same time, we are lead to see him as if he is not. He is something more sinister, the physical embodiment of something far more wicked.
For example The Joker has no past, no identity, nobody knows who he is or where he came from. His fingerprints don't match any from any database (notice he has fingerprints, he didn't burn them off to hide his identity or past), his DNA matches none in any database, and it's left at that. Unlike everyone else in the movie, he just is because he is. And how can you not be haunted by his final laugh at the end, as he hangs upside down? For one brief moment the sound is distorted (or heath ledger is just really awesome) to make his laugh sound inhuman, too deep, like satan or demons are often portrayed when speaking. The joker is human, but at the same time he represents, embodies something deeply inhuman.
I say inhuman because his goal is to portray humans as something primal, something animal, where survival trumps everything else. This doesn't apply to him. He doesn't live because he wants to, unlike everyone else he doesn't have this survival instinct. Death means nothing to him. He wants batman to kill him, he wants harvey dent to kill him. He is not motivated in the same way as humans are, he lacks anything that makes him human.
The joker, this demon that exists outside our human ethos takes the audience and gotham through a little thought experiment come alive. How does he do this? By creating scenarios and letting us watch what happens and question what we would do, what would our city do, our loved ones what would they do?
What is morality? As the Joker Defines it, morality is rules, rules that define a plan, and the plan is for us to be happy. Humans are happy as long as things go according to plan, as long as people are playing by the rules. The specifics of the rules don't matter, because every culture, group, or gang has it's own rules, and they hatch plans based on these rules. The joker doesn't see a difference between the mob and the cops, all of them define themselves by artificial boundries of what they think they will and won't do, what should and shouldn't be done.
What makes us moral? Some inborn desire to do "good"? Essentially to follow the rules out of belief (or misguided self righteousness as the joker puts it) that the rules are beneficial to everyone. That if we follow the rules, then things will go as planned and we will all be happy. Batman believes that heroes are needed to exemplify the rules, to enforce them, and be a beacon of hope. But that he isn't that, and that he can't be that. His is a struggle of idealism vs pragmatism. Harvey dent is the ideal. Some one that can play by the rules, and show that the institutions, and guidelines work, that the plan works and is beneficial for us all. Batman is the pragmatists result though. He is "whatever the people need him to be". Sometimes that means he doesn't play by the "rules". He does what's necessary, but he has his own morality, his own limits that he refuses to break because he believes what he's doing is fundamentally good. He desperately wants the ideal to win out of his pragmatic method.
What does it take for civilization to become immoral? As our guide through this experiment puts it "They're just a bunch of wild dogs. Once the chips are down they will eat each other alive". The joker believes that we are still fundamentally animals, and that our will to survive trumps any kind of "morality" or cultural rules. This demonized force that exists outside of our civil world seeks to expose how fragile and meaningless our boundaries are. All he has to do is apply a little bit of pressure. In the beginning he takes over a gang by killing it's leader, no doubt this group had it's own sense of honor. They fought for each other, protected each other, worked towards mutual benefit. The joker kills their boss, breaks a billiard cue and drops it in front of the three survivors, saying there is only one position available. Faced with death, their boundries and rules crumble and they savagely fight one another for survival.
I won't go through each example one by one, the theme is recurring as he tests the city and the individuals that act as icons for morality. He wants batman to kill him, and he wants harvey dent to break and become a fallen angel, a demon no better than Joker. At the end, the director hands the question off to us. In the boat, hundreds of ordinary people are asked to make a decision. Blow up the other boat killing hundreds of convicts and a few good people? Or wait to get blown up by them or the joker? By using hundreds of ordinary people, the question is asked of us "What would you do in this situation? Would you vote to kill hundreds of people? If it came down to it, could you push the button that would end their lives? Would your loved ones?" It's chilling, because deep down, we suspect that the joker is right.
