Several examples of genocide from Armenia, the Ukraine, and Rwanda, of systematic political slaughter (Cambodia), and of massacres in Nanking, My Lai, Viet Nam, and El Salvador are examined. Massacre typically occurs during wars, genocide, and political slaughter typically after a war has occurred and further conflict is feared. Political and historical factors shape the selection of a target group. One prominent feature is the belief that the target group obtained unfair advantage in the past. The social violence is then justified as revenge. Symbolic restructuring of the target group leads to their being viewed as viral or cancerous. This perception justifies the killing of nonviolent target group members on the basis of future risk. Whereas most genocides emphasize “efficient” slaughter, massacres are generally more cruel. Rape, torture, and mutilation typically precede killing. Many soldiers engage in these actions, although no information suggests they have propensities for rape, sexual sadism, or sadistic violence in civilian life. The extreme cruelty is therefore hard to explain using forensic trait theories. Social psychological theories of state-determined violence explain the transition to violence, if not the extremity observed. A suggestion is made for a form of forensic ethology that examines human actions in war settings based on survivor reports and tribunal transcripts and combines these with existing corroborative information on perpetrators.