S.T. Karnick teases out some more reasons The Dark Knight deserves its unprecedented box office success, including one aspect I loved in the film:
[The Dark Knight] powerfully brings out an important philosophical and theological idea about the nature of evil: that it is the absence of good. Evil is parasitic in not having any true nature of its own but simply functioning as the negation and destruction of what is good.
That is vividly evident in the Joker's lack of interest in obtaining anything good. Criminals, after all, typically commit their crimes in order to get things that we all consider good, such as money, power, gratification, etc. The Joker will have none of that. His every impulse is simply a nihilistic desire to destroy things, create disorder, and spread unhappiness.
Yet he does not appear to derive any real joy from it. In this the character differs strongly from other modern film villains: he is not a sadist, for he apparently does not derive pleasure from his actions. There is a certain powerful melancholy at the heart of the character which Ledger establishes brilliantly. (Note the photo at the top of this article.)
The Joker's wish regarding Batman, notably, is to bring him down to his level, to make the hero into the villan the Joker is sure the Batman really is in his heart of hearts.
Even his calling card, used several times in the film, adds to this meaning. It's a joker, of course, the card with no fixed value. Everything about the character suggests a deliberate and detailed personification of nihilism in its most sadistic form.
In this joyless impulse toward destruction, the Joker strongly evokes the Judeo-Christian conception of Satan. The Devil, after all, is an accuser, an adversary to humans, who wants to harm them both in this world and in the next. He is not a creator, however, but exclusively a destroyer.The Joker's passion for destruction, his insistence that others are just as corrupt as he but not honest about it, and his clear lack of joy in anything he does all make the character much more than just a cartoonish movie villain. For that, both Ledger and the Nolans deserve much credit.