Bruce Wayne is face with making the decision on whether or not to reveal himself as Batman to stop the Joker from killing innocent people.  The Joker isn't in it for the money, he is in it for the terror he can bring to people.  His motivation throughout the movie is fear.  The Joker gets into Batman's head by telling him he has the love of his life Rachel and District Attorney Harvey in separate buildings filled with explosives.  At this time all Batman can thing about is saving Rachel, but comes to realize he reached Harvey instead.