Murder is wrong because murder has a specific definition, and the society that we live in has defined that as wrong. A soldier killing a soldier is allowed during wartime (generally). We have judged that differently. There are a number of reasons why you might kill a person. Some are allowed, and some aren't. It is a fact that your society has a law prohibiting civilians committing murder. Whether the killing of a person is "right" or "wrong" by law is a moral judgement - not a fact - depending on who did it, how they did it, when it was done, and how your culture combines those factors. These tribes likely have similar beliefs, they just might be different to yours. Assuming yours is are the 'correct' ones, is arrogant.