The only sensible way to live in this world is without rules!
From the start the Joker pulls back the curtain on true evil, showing the world to be horrifying place when morality is thrown out the window and replaced with chaos, lawlessness and an ends-justify-the means mentality.
Other villains have come and gone, but the Joker should remain imprinted on people because he is evil with all the glamor and ambiguity stripped away. He lies about everything, he relishes in goading good people to break rules, he destroys things and people for fun and he does it all for evil's sake.
See, I'm not a monster...I'm just ahead of the curve.
"What is truth?" "What is good?" "Why do the good guys have to play by the rules?" are all questions that flesh out this movie into something far more serious than your typical action movie.
If there was an essay entitled "How to watch a movie," this would be the first one under the category of "Yes, it may be entertaining, but what's the message?" There are plenty of entertaining movies out there but you always have to be asking yourself "What's the movie trying to say?" If the message is corrupt, no entertainment value can redeem it. Think about how many movies glamorize crime. As entertaining Gone in Sixty Seconds was, there really isn't any redeeming value to the film. On the other hand, movies with good messages that lack good directing and acting are almost worse since they can turn people off to the message.
The Dark Knight manages to address the big issues of morality AND entertain. I hope that those who see it will talk more about what was said than about a semi-truck flipping end over end.
Because he's the hero Gotham deserves, but not the one it needs right now...and so we'll hunt him, because he can take it. Because he's not a hero. He's a silent guardian, a watchful protector...a dark knight.
So, what is good? How should people live? The movie looks at three different answers. The first, is the Joker's option - "a world without rules" where power is all that matters and civilization is only a veneer over the general evil at the heart of everyone. The Joker relishes in corrupting people and there are echoes of Christ's temptation in the desert as the Joker tries to at different times goad Batman into running over him with a motorcycle, pummel him to death or drop him off a building. The Joker is so repulsive that his answer to the question is easily dismissed.
You thought we could be decent men in an indecent world. But you were wrong; the world is cruel, and the only morality in a cruel world is chance.
I took Gotham's white knight, and lowered him to our level. It wasn't hard. Y'see, madness, as you know, is like gravity. All it takes is a little...push.
The second possible answer is given by Harvey Dent after he becomes Two-Face. He is broken by the death of his future fiance and the Joker taunts him into taking "justice" into his own hands by murdering the people who may or may not have been involved. He actually takes his justice one step further by flipping a coin to see who lives and dies, so you can't even say that he is a vigilante dealing out justice in a corrupt city. This view of the world is the godless, everything is chance view where there is no fixed morality, only random chance that determines people's fate. Again, this view of the world is dismissed as unacceptable because there ARE rules and there IS a fixed notion of good and evil.
You truly are incorruptible aren't you? You won't kill me out of some misplaced sense of self-righteousness, and I won't kill you, because you're just too much fun. I think you and I are destined to do this forever.
The final view of the world is Batman's view - right and wrong, good and evil are real things that can be bench marked. The Joker spends the whole movie trying to tempt Batman and others into violating morals and doesn't seem to understand why anyone wouldn't rather play by "the ends justify the means." Batman refuses to kill the Joker, or anyone else for that matter, because criminals deserve to have a trial and go to jail. True justice isn't killing those you think are guilty, true justice, even in a city where the justice system is corrupt, is to give criminals their day in court.
Another point made near the end of the movie is that you can't kill others in cold blood, even criminals, to save your own life.
Sometimes, truth isn't good enough, sometimes people deserve more. Sometimes people deserve to have their faith rewarded.
The final point that the movie makes that reminds of a line from Secondhand Lions is "Some things are worth believing in even when they aren't true." Some may object that this is tantamount to lying but what it really is is charity. Batman takes the fall at the end of the movie for Two-Face's rampage in order to preserve Harvey Dent's image as Gotham's "white knight." There isn't any good served by destroying the character of Harvey since he is dead. In a similar way, the recent attempts at deconstructing the Founding Fathers serves no purpose except to destroy the character of men who don't deserve to have their character destroyed.
All in all, The Dark Knight is one of the best morality plays to hit the screen in years. Hopefully the special effects won't overshadow the message of the movie.