We provide a model of conflict and mass killing decisions, to identify the key variables and situations that make mass killings more likely to occur. We predict that mass killings are most likely in countries with large amounts of natural resource rents, polarization, institutional constraints regarding rent sharing, and low productivity of labor. The role of resources like oil, gas and diamonds and other key determinants of mass killings is confirmed by our empirical results based on country level as well as ethnic group level analysis.