@Thomas,
I concede this point; All societies have acceptable killing and unacceptable killing.
But, there are some societies where I would be on the acceptable to kill list (interracial marriage, sexually active before marriage, big mouth on political matters, rejection of religion). I bet everyone is acceptable to kill in some cultural context.
Tribalism is the evolutionary trait behind our unwillingness to kill some people. There is survival value, particularly in social animals such as us, to form groups of people similar to us. We have an instinctual empathy and desire to protect people in our tribe.
If you want to call this a form of primitive moral value, Ok (I am not going to argue over terminology). But other primates, Gorillas and Chimpanzees for example, have the same moral value.