A unifying principle of American law governing the defensive use of deadly force is that a victim may use such force when an attacker threatens violence capable of causing death or grave bodily harm. There is little doubt that it is justified when the robber draws and points a handgun at the victim.
The act of self-defense can have a double effect: the preservation of one's life;
and the killing of the aggressor...The one is intended, the other is not.
— Saint Thomas Aquinas