Research on the consequences of social movements typically aims to identify determinants of success or to draw attention to ways that social movements are able to secure new benefits for constituents by gaining concessions from political authorities. Yet social movements, even those that are ultimately defeated, may have an enduring impact on the communities in which they were once active. This impact may be far removed from the movement's stated goals and may be detrimental to constituents and to society at large. We identify an empirical relationship between Ku Klux Klan activism in the 1960s and increased numbers of homicides in southern U.S. counties in subsequent decades. We explain this finding by drawing attention to ways in which right-wing extremism can disrupt community cohesion, generate mistrust in legal authorities, and promote interpretations of conflict and conflict resolution that weaken constraints on violent behavior. (Contains 5 notes and 3 tables.)