This chapter provides a rationalization of large-scale atrocities consistent with the main characteristics of the history of the last fifty years. Mass killings or genocides are planned; they are perpetrated by groups in power; they typically happen at the end of civil wars; they sometimes follow from external pressures or constraints on the group in power; the risk of mass killings is highest in polarized countries with low productivity and high dependence on natural resources. The normative implications in terms of third-party intervention have to be carefully considered. Neither a threat of direct intervention (effectively putting a cap on allowed mass killings) nor the imposition of minimum standards to be used for the treatment of defeated minorities can be evaluated in the absence of consideration of the economic structure and social divisions.