One more point here regarding American law is worth bringing up, and this is true in all 50 states: If people are throwing potentially lethal rocks at you, then you have no duty to retreat. Riot police sometimes might pull back anyways, to avoid escalating matters. But Border Patrol agents on patrol don’t have the kind of armor and shields that allow riot police some protection while pulling back. Under such circumstances they cannot safely retreat, and that being the case, deadly force is then lawful and allowed by their regulations to stop the attack.