The Batman has a pretty strong ethical code, and while he's doing what he has to do to protect Gotham City, he's not sure he likes having to break the law to do it. Harvey Dent looks much stronger, principle-wise, at least on paper, but as the movie goes on, those principles start to crumble. The cause? The Joker, of course, who's out to prove that principles of any sort are absolutely meaningless and that we should all feel free to commit murder and mayhem because who cares? In that sense, principles and ideals become the stakes in The Dark Knight, with Batman and his buddies doing everything they can to stick with them, and the Joker trying to prove that they don't really exist.
Questions About Principles
How do Harvey and Gordon stick to their principles as the movie goes on? How do they break them either deliberately or accidentally?
Does Batman truly have any principles besides his refusal to kill? What are they and how do they help him?
What methods does the Joker use to show how meaningless everyone's principles are?
How are the citizens of Gotham complicit in the loss of principles and ideals? At what points do they stand up and affirm those principles?
Chew on This
The good guys' sacrifices are worthwhile because they preserve the ideals they believe are so important.
The good guys save Gotham, but at the cost of their own principles, basically making the Joker's point for him.