It's the Joker we want to see in Batman movies. In "The Dark Knight", Heath Ledger's interpretation is as chilling as we might ever get. Ledger's Joker is no criminal; more like a violent, wanton, sadistic anarchist. He has no agenda beyond playing the role of the bringer of Chaos, and delights in being the spoiler of well-made plans. His aim is to bring an end to social order. Every system is an equal opportunity target, even organized crime. His theory is to eliminate those at the top and the people down the line will eventually turn on and destroy themselves.
The odd thing about this Joker is that he has no "origin" backstory. Just as his face hides behind that grotesque mask, his identity is a complete mystery. It's like he has no beginning and consequently, no end either. He could be anyone, and no one. He is thus the ideal metaphor for the unknown a paranoid nation fears, freaked out by terrorists hiding in every flitting shadow.
The Joker's machinations are sadistic experiments in how far he can push the people he places in mortal moral dilemmas to cross the line. And in Gotham City, so reliant on the Bat as their protector and guardian, and so sold on Harvey Dent's grand vision to turn its crime-ridden streets completely around, the Joker wants to know if the Gothamites have it in them to save themselves when they have no confidence left in their fragile social structures and systems once he's trashed them.
The Bat, rock-solid, honourable, courageous, in possession of a social conscience, and displaying a fighting style so efficient it's almost perfunctory, is every bit the warrior-hero we expect him to be. He too is an anarchist of a different sort. He upholds justice but is not constrained by the legal system. The anonymity behind the mask and a fat bank balance helps. But as the mask hides his face, his is not the face the public can trust.
The Bat, he's the devil we know. The Joker, he's the one we don't. Maniac, unpredictable, menacing, and with serously bad hair, he's the one who hooks us with horrid fascination in this movie. The Joker pulls the strings and the Bat has to play along. With his deft manipulations, Harvey Dent transforms from hero to monster. "The Dark Knight" is a vehicle for the Joker, the raison d'etre that brings his supporting cast to life. The Bat and Dent lose so much to the Joker's insanity. What did it cost Heath Ledger to bring this incarnation of the Joker to life?