Any character works better for the reader if they have a convincing back story, and there's never been a person in 'real' life who was purely good or bad.
My wife writes and teaches writing. She is big on the idea of a back story that must exist, even if the reader never knows about it. The actions of the character reveal it slowly through the book and as long as every action feeds off the back story and new developments in the plot, you've got a character the readers can relate to.
George RR Martin's character arcs for Tyrion and Jaime in 'A Song of Ice and Fire' are great examples.
The other great piece of advice I heard that relates to your search for greater understanding of other is to separate a person's actions from their character. A policeman told me that:- he didn't arrest bad people, just people who had broken a law.