You've equated war and murder, but the distinction between the two is one of the brightest lines in many law codes. Murder is a private act committed by private individuals, while war is a public act of friend against foe (distinguished as a public enemy in contrast to private ones).

Further, murder may be restricted to the killing of publicly acknowledged members of the public "friend" group, i.e. citizens, while the killing of outsiders living with the "friend" group, like slaves, is considered something else in the law.

When we codify morals as laws, we usually make a heavy and deliberate distinction between private and public, and between citizen and non-citizen. This is probably related to the nature of a social animal.