Go read this very nice essay on The Dark Knight over at Tor:
In fact, the whole film acts as a condemnation of vigilante justice. Batman is particularly bad at fighting the idea of crime. Sure, a transforming car and mad ninja skills are useful in a fight, but using these powers in the dark, without the rule of law or open conduct as a masked dictator (Batman is explicitly compared to Caesar) does nothing to support society and only feeds into the Joker’s plans. The Joker has an unlimited number of minions (including within the police department), Batman limits himself to a handful of allies. Batman inspires no one but idiots in hockey pads and the Joker himself, while the Joker brings out the worst in criminals, the general populace, and most obviously, in Gotham’s best defender, Harvey Dent. That Batman can’t save Harvey, in the end, is his ultimate failure: he can’t inspire good in even the best man he knows.
This acts as a nice counterbalance to all those blinkered essays back when the movie came out about how Batman is a fascist, promoting a reactionary conservative ideal of crime fighting. It does so, only if you assume, against the evidence presented in the film, that what Batman is doing is right. He’s not. He’s made a mistake and taken the idea of crime at face value. That it’s a Newtonian reactive system, where the criminal pushes and you push back harder. This is not so and Batman learns this the hard way, which is why at the end eh shoulders the burden of becoming the shadow for Gotham City. He will be hunted and brought low for his failure to act instead of reacting.
It will be interesting to see how this plays out in the third act of the trilogy, the Dark Knight Rises.