Why is the Joker more interesting than Batman?

Or why is Milton’s Satan more interesting as a character than his God? Or why is Dante’s Inferno more entertaining than his Paradiso? Why, in general, are villains more compelling in literature, while the good guys tend to seem stereotyped and bland? (Though, as C. S. Lewis points out, the opposite is the case in real life, with bad people tending to be monotonous and uninteresting in their self-possessed solipsism, while good people tend to be varied and enjoyable to be around.) The answer to these questions tells us much about the human condition.