Valerie’s use of defensive force has two good effects. It prevents the threat to her finger and prevents the opportunity cost of not saving Billy (on opportunity costs and defensive harm, see McMahan 2014: 4–6; Draper 2016: 109–115; Oberman 2019: 199–202). It seems plausible that preventing the opportunity cost can contribute to rendering harming Amanda proportionate: Valerie may impose more harm on Amanda than she would if only her finger were at stake. The difficult question is how much additional harm. Is Amanda liable to lethal harm, given that her attack endangers Billy’s life? This will depend on, among other things, whether there are differences between liability for causing harm and liability for preventing people from being saved.