I am inclined to think that calling it murder means that you are saying that it's morally wrong. I take it justifiable homicide is not murder. So "Murder is wrong" is true by definition. However, we can imagine a society that considerably expanded the range of justifiable homicide, and the Amazon culture seems to. I can imagine a society that considered it justifiable homicide if you killed a cheating spouse. If relativism is true, then that we could never say that that society was in error for allowing such homicides.