I think the best bit of advice I ever got about people and how we seen them was during officer training. Our instructor said "Yes, we like to think we're the good guys. But when you lead you must always remember that even good guys can do bad things. Think of people and their actions in terms of us and them, not in terms of good and evil. That way you can get down to case specifics in any given situation." I didn't quite see the point of that until I served on a UN mission in Africa and saw both sides of the conflict. That was when I understood what our instructor had meant. And I now apply the same sort of thinking to my characters when I write. None of my characters are good or evil in the conventional sense, they're just people who act in what they perceive to be their own and others best interests.