This article argues that revolutionary leaders are more willing to commit mass killing than nonrevolutionary leaders. Revolutionary leaders are more ideologically committed to transforming society, more risk tolerant, and more likely to view the use of violence as appropriate and effective. Furthermore, such leaders tend to command highly disciplined and loyal organizations, built in the course of revolutionary struggles, that can perpetrate mass killing. This study uses time series cross-sectional data from 1955 to 2004 to demonstrate that revolutionary leaders are more likely to initiate genocide or politicide than nonrevolutionary leaders. The violent behaviors of revolutionary leaders are not limited to the immediate postrevolutionary years but also occur later in their tenure. This demonstrates that the association of revolutionary leaders and mass killing is not simply indicative of postrevolutionary instability. This article also provides evidence for the importance of exclusionary ideologies in motivating revolutionary leaders to inflict massive violence.