It's Batman's thing, right? The hunt for justice and the ability to judge others for their crimes—it's what superheroes do. But in this case, he's got a whole cast of characters trying to do the same thing. The funky thing about The Dark Knight is that it's not just limited to the good guys.
The Joker also wants justice, but only in a sick, subversive, totally monstrous way. Chaos, to him, is the only just thing in the world, and by promoting chaos, he's doing away with all these flawed complicating structures (like the legal system) that clearly aren't getting the job done. It's messed up, but has a certain insane logic to it. And to quote Maroni again, "have you met this guy?"
Questions About Justice and Judgment
Can you spot signs of Two-Face's twisted sense of justice before he becomes Two-Face? If so, where?
What, exactly, gives Bruce Wayne the right to seek justice in the manner he does? Is it better or worse than the methods used by Dent and Gordon?
Why do you think the prisoner in the ferry throws the detonator out the window? What does it say about justice when coming from someone like that?
Do Batman and Gordon truly bring justice to Gotham at the end?
Chew on This
Justice can only come when laws are strictly adhered to and applied without bias.
Every system is flawed, and justice is possible only when people know when and where to set aside the rules.