How does the way in which a group organizes change the lethality of the group’s attacks? In this paper, we argue that groups organized vertically as hierarchies are likely to conduct more lethal attacks. We build our argument around three advantages inherent to centralized structures: functional differentiation, clear command and control structures, and accountability. We argue that each of these characteristics positively impacts an organization’s ability to deliver an effective lethal blow. To test our argument, we use a mixed method approach, drawing on empirical evidence and support from a time-series case study. Our large-N analysis examines the trends in more than 19,000 attacks. In this test we develop a novel proxy measure for hierarchy based on a group’s bases of operation and non-violent activities. To complement the empirical work, we examine the history of Euskadi Ta Askatasuna (ETA), the Basque separatist group. Over several decades of violent operations, this group’s structure has changed dramatically. We analyze how these shifts impacted ETA’s ability to maximize the effectiveness and damage of their attacks. In both the case study and large-N analysis, the more hierarchically organized the group, the more easily the group can orchestrate lethal attacks.