The Dark Knight
All of the Joker's from the 1960's TV Batman onward were crazed but this one was frightening in a more philosophical way. He was a source of pure destructive chaos and nothing more. As one character put it "His only desire is to see the world burn." Not only did he not care for the lives of others, but he little regard for his own life which made him utterly unpredictable and dangerous. This,along with his intelligence and a dark understanding of human nature, combined to create an almost perfect bad guy. Most villains have some redeeming features but the Joker has none, except that he at least appears to be totally consistent with his mad philosophy.
His philosophy, such as it was, said that goodness was an illusion and when forced to make a choice, all humans would revert to mere animals fighting to survive. His belief in this idea was total and if he had a goal at all, it was to prove his belief true, no matter how many lives were destroyed in the process.
The movie gave no background for the Joker. He was simply there.
It came down to this. Batman was not only fighting the Joker, he was fighting the insane philosophy that motivated him, not only in himself but in those around him as he struggled to capture the human(?) fiend.
Much can be read into this but he reminds me of another force of pure chaos from the seventy to ninety year old writings of a slightly deranged science fiction writer named Howard Phillips Lovecraft. He created a god of sorts whose name was Nyarlathotep who appeared in several of his stories and whose nickname was The Crawling Chaos. He was a personification of the cold, uncaring, impersonal universe who lived for no reason but to build and destroy for no purpose whatsoever, except for that is was he did.
In a bizarre way, the dilemma offered by the Joker is really another question. Is there a God Who sets the standards of right and wrong? The question is never actually asked but is intrinsic to the Joker's actual question. Can there be a true morality without God? If so what is its foundation? Or do we go to the logical conclusion of the villain of another work, Dostoevsky's The Brothers Karamazov who decides that its OK to murder because if there is no God then all is permitted. How do you prove moral authority to one who does not believe in any form of authority. To those who believe in God it is not a question, but to Atheists and Agnostics it is a serious dilemma. That is what makes the Joker so scary. He personifies the logical conclusion of there being no final moral authority in the universe.