Us and Them: Moral Instincts
A fundamental challenge of human nature is that our fellow humans are not only our primary evolutionary competitors but also essential for our survival. This tension probably has a lot to do with our instincts about morality and behavior. Most of the time, we need to do some social calculations concerning whom is with us and whom is against: "us" and "them". It's complicated because those boundaries shift a lot.
My guess is that our moral instincts are mostly about how we manage those calculations.