Well I would first give him the opportunity to to pass out or tap out! My arm around his neck till his air or blood flow stopped and he was out for the count. That not working a 9mm to the head!!
There is not question of if it is a shoot/no shoot there is a life in danger; that is why the PO shot the guy, or that appears in danger (if the child is dead I do not know this as a fact till he is out or dead).