Scholarship on De Officiis tends to place the emphasis on those aspects of the treatise that outline the building blocks of a functioning commonwealth. What tends to receive rather less attention is Cicero’s theorizing of what to do in situations of communal breakdown. But in De Officiis he is as interested in injustice as he is in justice, in the ability of humans to inflict massive damage on each other, in the insufficiency of legislation to ensure harmonious co-existence, in fraud and abuse of power. To address the problem of harm and injustice Cicero calls for vigilance and intervention on the part of all members of the community and endorses ‘protective violence’ up to and including the license, indeed the duty, to kill tyrannical figures that threaten the fabric of communal life. This chapter argues that Cicero’s ethical extremism animates the entire treatise. To render this claim plausible it explores the historical circumstances that gave rise to it and traces its ubiquitous presence within De Officiis, with a particular focus on the conceptual strategies used to justify murder.