We are a supremely social species whose ecological success rests largely on our capacity for large-scale cooperation. This high degree of sociality is only possible against a background of immensely powerful inhibitions against performing acts of lethal violence against conspecifics. There are circumstances, however, in which acts of lethal violence are individually or collectively advantageous and attractive. To perform such acts, we must override our inhibitions. This essay argues that this tension causes us to be ambivalent about killing other human beings and that our being so is manifested in the widespread belief, found across cultures and historical epochs, that taking human life contaminates the killer and may pose a threat to the entire community, unless rituals of purification are performed to counteract it. Examples from the Hebrew Bible, the Greco-Roman world, medieval Europe, Africa, and Native America are examined to substantiate this claim. Premodern beliefs, moreover, about the consequences of killing are echoed in the symptoms of “moral injury” described by contemporary psychiatrists treating combat veterans, which suggests that, in defying or disabling our inhibitions against performing acts of lethal violence, we ultimately do violence to ourselves.