people can evaluate their behavior either by relative yardsticks - law and morals of the group/society they belong too - or by absolute yardsticks of good and evil, like don't kill, etc... Using the relative ones is easier and gets you comfortably to the advanced age. Thus evolutionary the relative ones prevail inside given group/society. There is though some kind of correctional feedback at work that when the relative yardsticks of a given group/society drift too far from the absolute ones, the whole group/society is eliminated (or some kind of catastrophically adjusted) from the race.