This paper focuses on this pattern in the self-styled Islamic State’s (IS) campaign against the Yazidis, a religious community with historical roots in the Sinjar area of northern Iraq in the summer of 2014. Thousands of Yazidis were summarily executed; large numbers of women and children were taken hostages and subsequently sold as slaves. What explains the IS’s extraordinary violence against the Yazidis? What were the motives of individuals who took part in the atrocities? This paper argues that one needs to look beyond Islamic state’s ideology and opportunistic motives to deep-rooted stigmatization of the Yazidis to explain the patterns of complicity and participation by the local population in the violence. Preexisting religious tensions and prejudices at the local level have played a strong role in shaping local population’s participation in the genocidal campaign. This explanation emphasizes the importance of the historical substance of ethnic identity (defined through religious beliefs and markers in this case) in shaping inter-communal relations. It also shows that alternative theoretical frameworks based on the notions of security dilemma, resentment, and greed do not provide satisfactory explanations of this pattern. The paper combines several different qualitative methods and is based on several original empirical sources. First, the authors have been conducting dozens of face-to-face in-depth interviews with Yazidi leaders including Baba Sheik, the top-religious authority of the community, and survivors including women who were taken captive by IS for extended periods of time in Iraqi Kurdistan (two different visits in September 2017 and May 2018) and Germany (June 2017). The authors use these interviews to obtain information about (a) the nature of inter-communal relations at the local level before the attacks, (b) the behavior of non-Yazidi neighbors during and in the aftermath of the attacks, and (c) the Yazidi perceptions of the causes of the attacks. These interviews develop a comprehensive view of the Yazidi population with regards to social caste, economic class, gender, age, and geographical location. The authors supplement these interviews with recorded testimonies of survivors conducted by local researchers and field reports by international associations and local organizations. Additionally, the authors have been conducting in-depth interviews with activists, professionals, and local authorities involved in the Yazidi affairs. Finally, the authors compile a rich set of historical and contemporary documents in primary languages (i.e., Kurmanjî and Sorani dialects of Kurdish, Arabic, and Turkish) about the perception of Yazidis by non-Yazidi communities and ruling authorities.