Warning - Spoiler alert! if you still haven't seen the movie, you might want to skip the rest of this until you have done so.
What makes the Joker so interesting, almost appealing? He is an intriguing character. Throughout the movie, Joker's enemies try to know and understand him. They fail miserably. He has no motive, no plan. He just does, impulsively and erratically.
Is this insanity? Almost all people would agree, yes, he is insane. What is insanity? The basic definition is abnormal behavior, or something similar. So, the Joker is not like most people. Most people want money, fame, or power, as our society tends to preach the values of those incentives. As mentioned previously, the Joker wants none of these things. He only wants the thrill of the chase. Is this very far from abnormal behavior? Every human has bloodlust, but 'normal' people keep it hidden. Other people just go hunting. How far a leap is it from deer to humans? One cannot deny that it is a significant jump, but the underlying behavioral mechanisms are the same.
The Joker, on some level, represents an unleashing of human desire on a very instinctual level which is normally kept hidden according to society's rules. He is such an interesting character because the audience identifies with him in a way which is unsettling. In contrast, the Batman seems totally lacking in human instincts. He sacrifices his reputation so the people of the city can maintain hope for the future, and never find out that the good guy of the city (Harvey Dent) went bad in the end. Every human has a need to be liked and appreciated. Batman totally abandons this instinct. The audience does not identify with him at all; instead, he becomes a figure of moral purity, letting go of his humanity and becoming a social hero instead. He is untouchable to the everyman, with his far higher standard of morality. The Joker, instead, is everyman, but completely uninhibited about his instinctual desires.
Perhaps, dear reader, the hero of the movie is the insane one. Enjoy your meal.
