It is our irrational part, at least that is how I see whole good-bad thing. I consider myself good and would help fellow human if I can. That includes saving from being eaten by another human.
Good usually comes with righteous. Which means I would gladly kill cannibals, even though they are fellow people. Except to me they are not, anymore, if they do harm people. You know tolerance, you're free to do anything until you take another person's freedom. Then you're not free at all. Many people around the world share same mindset and that is why crime and killing fellow humans tend to get punished.