And the answer is when it becomes human.
Humans killed 400+ civilians at My Lai, and 200+ civilians at No Gun Ri. Both massacres were the result of rage and fear. Robots don't feel those emotions, and have committed no massacres on that scale. I trust robots more than I trust humans.
Restrictions on nuclear warheads, ships, etc. make sense because they can be verified. Restrictions on software have no means of verification, so any ban on autonomous robots is wishful thinking.