The dark side is not only for two-dimensional villains and vampires. It's part of everyone in real life who has lived more than a couple of years. To express that in a character—whether villain or hero—to tap into the power that comes from showing truths about the character's deepest self, and thereby creating a complex, conflicted character, is one of the biggest challenges writers face. Why? Well, there are probably lots of answers to that particular question, but I'm going to suggest that it's because our "deepest self," warts and all, is something most people are uncomfortable examining, much less expressing for public consumption.
Via Ruth Long