The dynamic between good and evil has always been an important part in dramatic entertainment. God versus Satan. Moral versus immoral. Hero versus villain. Usually, an author will write their story/screenplay/etc from the hero's point of view, to assist the audience in associating with that character. White hats signify the courageous cowboy while black hats help identify the criminal. It is a tried and true formula, and one which continues to be successful.
But an interesting thing happens when we engage in quality entertainment. The good versus evil struggle might appeal to us at first, but it wears off the longer we stay engaged. The "good" characters we tend to enjoy are usually shades of gray. Hamlet. Han Solo. "Stone Cold" Steve Austin. The darkness, which we are asked to root against, makes the pure and good hero much more murky and "human."
As an audience, we understand that "humanity" as something that we all possess, and thus we are able to associate ourselves with these characters. Troubled but with a heart of gold. The vampire seeking redemption. Succeeding for right by any means necessary. Machiavellian and real.
And yet, who do we remember the most? The true villain. The one without redeeming qualities. The more over the top the baddie, the more we remember him or her. Darth Vader. Hannibal Lector. Cruella DeVil. Pure good seems boring to us. The heroes only become interesting once they are corrupted in some manner. But the villains... The worse they are, the more we remember them. Our shadows overwhelm our memory to clearly recall the horrible.