"The fact that governments can circumscribe the killings they do as “not murder” and have regular people believe them is the quintessential expression of the state monopoly on violence."

A state, and words and concepts, only exist insofar as people believe in them. You're only saying "murder is [defined]", not anything about governments. If you pretend you are making a statement about government or politics, it suggests that somehow a different system could achieve a better and different state. Everyone defining legal killing for themselves is not better or worse, it's a contradiction in terms - an absurdity.