Merely in virtue of our basic moral standing, all human beings enjoy a right to life. The protection afforded by this right can sometimes be lost—when a person becomes liable to be killed. But killing a nonliable person always wrongs that person, and is typically very difficult to justify. The first part of this paper argues that other things equal killing a nonliable person is more wrongful when the risk that you kill a nonliable person is greater. If wrongfulness is relative to the killer's beliefs, or the evidence available when he acts, then this proposition is uncontroversial. I defend the more controversial view that it is true in light of all the facts.
Applications of this proposition, if true, would be broad. But it is especially relevant to justifying the principle of distinction between combatants and noncombatants—the single most important ethical constraint on killing in war. Specifically, if noncombatants are less likely than combatants to be liable to be killed, it would follow that other things equal killing noncombatants is more wrongful than killing combatants.