On Violence and Tyranny examines historiography as a vehicle for the production of a theory of tyrannicide in the aftermath of the murder of Pedro I de Castilla (1369). The thesis of this work is that by considering the royal chronicle as a vehicle and locus for political theorization, we can appreciate the formulation of a theory of tyrannicide as a medium for dynastic legitimation that is not reducible to political propaganda. Rather, it becomes a meditation about monarchy itself, the limits of power, and the underlying causes and consequences of political violence. The chronicle of the king Pedro's rule conceives an economy of violence coded in terms of saber (political wisdom), justice and the law, as a means to face the ideological, political, and social challenges that civil war and regicide pose to a community. I will focus on two fragments of the chronicle, a pair of letters attributed to a wise Moor that the chronicler chose to include in a second stage of his composition and that establish extra textual connections to other political genres such as the specula principum and political prophecy. Through them, I will explore how a theory of tyrannicide allows the chronicler to confront three major problems that regicide poses. First, how to explicate the dynastic break that king Pedro’s murder brought about, and minimize the discontinuity that the advent of a new, and illegitimate, dynasty (the Trastámaras) represented for a historical tradition that deeply valued the continuity of history. Second, how a theory of tyrannicide served to repair the broken ties provoked by the civil war. And third, how to represent that founding violence, the violence against a sovereign, to render it legitimate, but not available for anyone else to exploit.