How do the perpetrators of mass killing legitimise their behaviour? This article examines the legitimation of some of the worst cases of mass killing in the past two centuries. It finds that the colonial experience helped establish a moral framework that facilitated arguments designed to place whole groups beyond normal legal and moral protection on account of some assigned traits. This moral framework was evident in different colonial settings and rested on claims negating the right of the victim group to protection and claims valourising their violent extermination. It also underpinned the moral justifications offered by perpetrators of some of the twentieth century's worst episodes of mass killing. This article examines the “family resemblances” between the arguments used by perpetrators in different settings, indentifies their common structure, and examines the factors that influenced their capacity to secure legitimacy for mass killing.