From author Linda Gray on Write of Passage:
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.
. . . .
The most riveting characters in literature are the deeply conflicted ones—the ones who are driven to behave in certain ways that go against the grain of what they know to be upright and good, for example. Anti-heroes are classic examples of that type of inner conflict. Think Scarlett O’Hara in Gone With the Wind and Rick of Rick’s Café Americain in Casablanca.
At least Scarlett and Rick get a chance for redemption at the end of their stories. But then there’s the even more complex character, the dark tragic hero who honest-to-god believes himself to be someone who strives to do what’s noble (and makes sure the world sees him that way), but is, in fact, driven by lower orders of need and desire.
Link to the rest at Write of Passage