Somehow, I am not having much of an ethical problem with killing those whose only goal in life is to murder innocents in order to foment terror. I don’t care if we eliminate terrorists by using drones, snipers, hand grenades, or nuclear weapons.
The potential for misuse of this technology remains. Nip gave several reasons in post (#3).
I’ll give another: If, in the hypothetical future, American human soldiers are told to fire upon conservatives, they may balk at firing against their own people.
Robot soldiers, having neither emotions nor a conscience, would have no scruples.