Well, I look at it like this. An evil act is an act that harms people to help or give joy to oneself - it's a selfish and cruel act. A good act is an act that helps people, and may or may not help or give joy to oneself - it's a possible selfish but kind act. We're all selfish, we all do things because they make us feel good, but what is it that makes us feel good? Kindness or cruelness?