Does this apply to soldiers in wartime? Most societies will not prosecute one of their soldiers who kills another human on the battlefield. Is this murder? Does your state make it illegal for a soldier to kill in wartime?

Many countries have eliminated the death-penalty, but some retain it. Is the executioner a murderer for a state-sponsored execution?

Most states have laws allowing violence if you're in immediate danger. If you kill someone in your state is that murder?

Other posters have already brought up some good examples to chew on, if it all seems so simple to you perhaps you’re oversimplifying things.

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.

Try reading "The Story of B". It discusses this exact circumstance.

Murdering people by definition is wrong, but it is not murder every time a person is killed by another person.

If someone murders someone in the us, they can get the death penalty. If someone kills someone in self defense, that’s not murder.

These people have government of a kind and a moral and ethical system. If the tribe kills someone within their own cultural traditions, that’s not murder.

Once they’re contacted and brought into a larger society, of course, that changes, but that’s a process that needs to be negotiated.