Even when the use of force satisfies the proportionality requirement, it may nonetheless be morally impermissible. For example, if Victim can avert a lethal threat only by either killing Attacker or breaking Attacker’s arm, it would be wrong to kill Attacker. Though proportionate, killing would be unnecessary in this case. As standardly understood, the necessity constraint requires defenders to compare the available means of averting a threat, rank them according to some relevant moral criteria, and use only the means that is favored by that ranking.