The Joker had an arc in TDK. At the start of the movie he is only interested in money, in stealing from the mob, and in killing Batman. He progresses to having a specific ideology (anarchy and the corruption of Gotham), he wants Batman kept alive, and he gives up on money in the amazing scene where he burns the mountain of dollar bills.
These changes occur because he realises how much fun Batman is, in and around the interrogation scene.
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He completes his arc later on when he calls the TV show and says "if that man is still alive in one hour, I will blow up a major hospital".
At first he wants to kill Harvey Dent just because it seems like the obvious thing for a criminal to do, but instead he chooses to corrupt Harvey Dent, to seduce him into a life of crime.
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That was part of the appeal of the movie for me, Joker was getting crazier and crazier.
That's if you believe Joker was telling the truth to the mob in the beginning of the film. Which I don't think he was.