This is from slashfilm and best sums up why it only a portion and the rest is here
http://www.slashfilm.com/2008/07/20/assessing-the-themes-of-the-dark-knight/
The Symbology of Batman
The final monologue that Commissioner Gordon brings the themes from
Batman Begins to their logical conclusion: Namely, that as a man, Bruce Waynes powers to evil crime are rather limited. As a man, he can be corrupted, he can be killed, and ultimately, he can be defeated. As a symbol he can become far more, and at the end of
The Dark Knight, he becomes, to society, an uncontainable force in very much the same way the Joker was. He becomes hunted, making people believe that he cannot be controlled, that he has lost all respect for societal norms and the rule of law. As Gordon realizes he needs to blame the murders on Batman, he acknowledges not only the need for society to push their fears onto something, but their hopes as well (which he allows them to do by preserving Dents good name).
In order to keep from tearing itself to shreds, society needs to believe in the incorruptibility of good and the relative remoteness of evil.
The Dark Knight points us to ways in which we cope with this need.
Simultaneously, its also made clear that, in fact, Batman never succumbs to his own dark, inner urges. In the movie, Bruce Wayne says the line, Ive seen what I have to become to fight men like him, and he rejects the path he has to take to stop Joker, a man who has no rules whatsoever. In one of the more memorable scenes from the film, the two have a showdown in Gothams city streets, the Joker manically screaming Hit me! as Batman is propelled towards him in the bat pod. As much as Batman wants to annihilate the Joker, he knows he cant violate his own moral code, and almost sacrifices himself to prevent this from happening (albeit as part of a broader ruse to capture him). Still, Batman doesnt seek to kill evildoers, but to bring them to justice. The dichotomy that the film sets up between Joker and Batman is one of chaos vs. order. The dichotomy between Joker and Dent is one of good vs. evil