If the structural conditions that increase the likelihood of participating in violence are so common, why do so few people participate in violence? What differentiates people who participate in violence from many more ostensibly similar people who do not? This research proposal outlines an individual-level theory of participation in violence based on emotional responses violent trauma, and the effect of contextual variables like justice and feasibility on emotional processes. People who experience violent trauma (like the murder of a family member) should respond with anger. Angry individuals should be likely to participate in violence in order to punish the perpetrator of violent trauma if a) they believe that punishment is feasible, and b) no third party provides justice on their behalf. The behavioral implications and emotional mechanisms of this theory will be tested using a series of structured interviews with family members of homicide victims in Chicago, Illinois. Data collection following this pre-analysis plan will occur between 2 January 2018 and 4 February 2018 in Chicago, Illinois. Interview questions were approved by the MIT Committee on the Use of Humans as Experimental Subjects, in protocol number 1707023191.