Abstract Genocide and mass violence originate in difficult life conditions, conflict between groups, and cultural characteristics such as a history of devaluation of a group, victimization, and overly strong respect for authority. These can join in creating uncertainty and fear, frustrating the fulfillment of basic psychological needs, and shaping destructive psychological reactions and social processes such as scapegoating and destructive ideologies. The evolution of increasing hostility and violence can follow, allowed by the passivity of internal and external bystanders. Halting genocide and mass violence is very difficult. It is more effective to focus on early prevention: responding to difficult life conditions, developing positive attitudes and constructive ideologies that humanize the “other”; dialogue; healing wounds and memories of past victimization; training about the roots, psychological impact, and prevention of violence in workshops and the media; and supporting development practices and democratization. Early prevention requires leadership in the United Nations, the work of NGOs, cooperating national governments, and citizen groups of active bystanders.