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I would argue that's because murder is generally understood to be tangential to state authority where state is defined as the monopoly on the legitimate use of violence. Killing for the state is war or exercising sovereignty or whatever the reason is, but it's the state's reason and it's weird to call state sanctioned genocide murder even when you acknowledge it as evil and unlawful. Killing against state authority is revolutionary action and while inherently unlawful is also rarely seen as murder. So it makes sense that a state sactioning the killing of actors of another state isn't seen as murder and instead has its own term for the whole tragic situation.