Why I'm a Kind of Moral Relativist

The author of The Splintered Mind wrote that he disagrees with Descriptive Moral Relativism and Metaethical Moral Relativism and he thinks that there are more shared ethical values of different cultures than ethical values that exclusively exist in a particular culture. Every sane ethnologist would agree that this is a very narrow point of view. But nevertheless I think it is partly true if you look at a culture which have achieved a certain level of development and if you attach priorities to different values, e.g. normally you would say it is a bit more offensive to kill your neighbour than peeing in public.
Why is it bad to kill people and good to hug them (if they want a hug)? Some of these core values can even be found in primitive cultures. Not for ethical reasons, but because of biological necessity. It is really bad to kill one of your tribe and to be cast out of society - for you! Your very own existence will be endangered - if you get away with the murder - since you don't have the security provided by the society any longer. There are a lot of other essential reasons not to commit murder, like the problem of gathering food and losing a safe place to sleep. But these core values don't exist in all cultures on our globe, "some societies allow their members to brutally kill the children of neighboring tribes or to torture crime suspects." (Waldmann 2006)