From a November 2015 interview with Big Think (video below):
“Purely good, purely evil character are as dull as dishwater. Superman is intrinsically boring. Darth Vader is intrinsically boring until a fairly clunky extra layer gets added later on in the character arc.
To truly animate a character, your angels need demonic flecks in them. Similarly, to truly make a malign predator, interesting on the page and followable and more intriguing, then that demon needs compartments within him or her that are not demonic, that are more angelic, that at least contain the possibility of redemption. When you do this, you’re making these characters more like the human beings we know from our daily lives, and good things happen to our narrative.
You want to follow them more. You care more. It’s about making a reader, and you cannot make a reader care about an archetype. They have to be fully fleshed with the same shades of gray, the same contradictions, the same sense that they’re not just one personality, but a multiplicity of personalities. We’re all a kind of colony, I believe, not of one, but of a dominant personality who calls the shots most of the time, but that’s not the whole story. There are others in there as well. Every Dr. Jekyll needs a Mr. Hyde. They’re there whether you want them to be or not, so why not invite them in when you’re making fictional people and allow them an influence? It makes the fiction better. It’s that simple.”