Finally, I think the characters in the movie exhibit a huge misunderstanding that is only revealed to them in the end. Dent was never the white knight. Or, rather, the white knight was never what was needed. In the end, Dent was supremely corruptible. Rather than become a true warrior for justice at the death of a loved one (like Wayne), he became chaotic, warping justice in the image of unguided chance. Batman was always the incorruptible one. The dark knight was what was needed all along. That is why I find it odd that they felt they needed Den’s image to remain spotless, even if it meant bad things for Batman. Batman was always the source of hope and proved to be the enduring ground of hope. But in making him the scapegoat, it seems they have undermined the role that he could play. Why wasn’t the dark knight good enough to fill the role of the white knight?

check it out.
http://vox-nova.com/2008/07/18/vox-nova-at-the-movies-the-dark-knight/#more-2949