Part of doing good means feeling obligated to do good: to set your personal wishes aside for the sake of someone else. In The Dark Knight, Bruce Wayne could easily just drop the whole thing and go party with supermodels on his yacht. But he doesn't because he can't abandon Gotham to crime and despair. Harvey Dent and Jim Gordon believe the same things, and together, the three of them won't rest until Gotham can sleep safer at night. The Joker finds ways to twist that around on them in fairly horrifying ways: seeing how far they'll go for the sake of their duty. Turns out, it's farther than any of them thought possible.
Questions About Duty
Unlike the comic-book version of Batman, Bruce Wayne wants to give up being Batman here. Does that make him less duty-bound, or just less crazy?
At what point does Harvey abandon his duty? What does that say about the connection between duty and the ability to do good?
How do the heroes' various attempts to do their duty result in positive effects for the city? How do they result in negative effects?
Does the Joker have a duty that compels him to act? If so, what is it and why would he feel bound to it?
Chew on This
Duty is ultimately a positive force in this world, driving heroes forward when they would rather give up.
Duty is only as positive as the goals it promotes, meaning that it can be used to push someone like the Joker to commit great evil.