Examination of detailed events data from the Guatemalan civil war between 1976 and 1985 reveal a complex, M-shaped relationship between ethnic mix and violence. This paper seeks to shed light on this seemingly mysterious relationship by presenting an agent-based model of civil violence that is inspired by an analytical approach developed by Kalyvas. This conception takes incumbent and insurgent levels of control as the main determinants of the level of violence. Violent acts are seen as a joint product of individual incumbent and insurgent actor decisions involving loyalty, defection, denunciation and counter-denunciation. They result from two conceptually distinct mechanisms: one that drives selective violence and another that drives indiscriminate violence. This paper focuses on the selective violence mechanism. The agent-based version of the model is able to reproduce the relationship observed in Guatemala more closely than its analytical predecessor. This study finds that the agent method is well suited to this approach to thinking about violence, allowing more flexibility in model construction and producing better qualitative fidelity to data, while retaining the same basic set of motivations. The model makes advances on two fronts. Firstly, it demonstrates how an agent-based model can be docked with an analytic model and then modified to better capture the path dependent, positive-feedback driven environment characteristic of civil violence. Secondly, it makes a direct link between a theoretical model and spatially explicit events data from a real conflict.