The 9/11 attacks, as well as the ones in Madrid, London, Paris and Brussels; the genocides in Nazi Germany, Rwanda and Cambodia; the torture in dictatorial regimes; the wars in former Yugoslavia, Syria and Iraq and currently in Ukraine; the sexual violence during periods of conflict, all make us wonder: why would anyone do something like that? Who are these people? Drawing on 30 years of research, in this book Alette Smeulers explores the perpetrators of mass atrocities such as war crimes, crimes against humanity, genocide and terrorism. Examining questions of why people kill and torture and how mass atrocities can be explained, Smeulers presents a typology of perpetrators, with different ranks, roles and motives.