The problem with "all war is murder" is that it tends in practice to be interpreted to mean that any attempts to pout a limit on the murder is futile, and undermines the concept of war crimes.

Trying to construct agreed limits is frustrating, and has a risk of appearing to legitimise terrible things - but the alternative is even worse.

War crimes include such practices as killing prisoners, targetting civilians, using civilian prisoners as shields, torturing captives, ethnic cleansing, and other things like that.

Outlaw them and war is still horrible. But at least in wars where people in power knew that they would have to answer in court for it, even if their side won, we might get away from a world in which the vast majority of people killed in wars are non-combatant civilians of all ages.