As written by nolan and goyer, Dent- and not Bruce wayne - would be TDK's tragic hero. "Bruce was the protagonist of the first film" explained Goyer, " but we decided very early on that he would NOT be the protagonist of the second film - that in fact, Harvey Dent would be."
One of the defining characterists of a protagonist is that he changes in the course of a story. Having determined that Bruce Wayne would not be the protagonist of TDK, the writers purposely constructed a story line in which the character remains UNchanged. "the character that changes is Harvey Dent. He goes from being a White KNight to a horrifically scarred and tragic figure"
'The battle for that tragic figures soul became the story's thematic thru-line. "We had Batman/Bruce on one side, saying that Harvey Dent represents a good vision of what the city can become" said Goyer "and then we had the Joker on the other side, saying that ANY man can be corrupted and turned into a villain if you push him hard enough. And the Joker is kind of right, which is why we wrote an ending in which Batman and Gordon conspire to hide the trust from Gotham. They protect the image of Harvey Dent and Batman sacrafices himself. That makes the ending of TDK very tragic"