Like the three branches of government—executive, legislative, and judicial—Gotham City has three branches to keep order in the city: the police, the mayor, and the mafia.
It's a little imbalanced.
The reason it's imbalanced is that the police and the criminals in Gotham are pretty much in cahoots, and the mayor is an idiot. We love saying "cahoots," but in this case, cahoots can be deadly for the peaceful citizens of Gotham. There is nothing keeping the bad guys in check. Nothing, that is, until Batman swoops in.
Batman fights to restore peace to the city with his own brand of vigilante justice. But by throwing the balance of power out of whack, Batman opens up a slot for someone else to tip the scales yet again. That's where Joker steps in to cast his judgment on Gotham City, a place he sees as the perfect playground of corruption for his antics.
Questions About Justice and Judgment
Why does Gotham City have a crime problem?
Does Batman bring justice to the city, or does he cause more problems than he solves?
Does Gotham City need Batman? What would the city be like without him?
What is Harvey Dent's job? Why is he in this movie? Does he do anything to help the city?
Chew on This
As a vigilante, Batman stops crime directly. He cannot work with the police because of corruption in the police department. But after cutting off their ties to the crime families of the city, Batman feels he can work with the police and presents them with the Bat Signal.
Joker casts his judgment on Gotham City. He sees it as an inherently corrupt place, and he uses it as the canvas for his anarchistic art.