Judy E. Gaughan writes in her book Murder Was Not a Crime: Homicide and Power in the Roman Republic:
During the Roman republic murder was not a crime. In other words, the “killing of a human being by another with malice aforethought” was not “an act done in violation of those duties which an individual owes to the community and for the breach of which the law has provided that the offender shall make satisfaction to the public."
Indeed, the republican Romans had neither the capacity nor the inclination to make the essentially private act of malicious and intentional homicide an offense actionable by the government. This fact is closely linked to the nature and evolution of political power in Rome, in large part because the right to kill is embedded in two key definitions of power: patria potestas (the power possessed by a Roman father over his children, which included the vitae necisque potestas, the power of life and death) and magisterial imperium (the power to command, which included the power to kill Roman citizens).
In this book I explore the relationship between homicide and power, with special emphasis on political power, from the beginning of the monarchy (753 B.C.e.) through the dictatorship of Lucius Cornelius Sulla (79 B.C.e.). The treatment of homicide, as revealed in this investigation of legislation, trials, punishment, assassinations, proscriptions, and the vitae necisque potestas, is a reflection of the extent and nature of the power of Roman government.
This means that when the treatment of homicide changed, it was symptomatic of a change in the extent of political power possessed by the republican government. Change in the extent of political power usually coincided with a change in the structure of government. These changes largely revolve around the extent of the centralization of authority in the government. Roman republican government had little inter- est in controlling murder because the government was too decentralized to have its power challenged by an act of murder. For most of the republic, the government did not have the capacity to involve itself in matters that were not of primary interest to its security and stability.