Killing people was not always wrong. Enemies were killed lawfully in war; the outlaw could be killed out of hand, as could the adulterer and the thief, provided certain conditions were met. Killing in self-defence was an accepted and universal justification, although the killer might have to run the risk of proving his case in a court of law. As the political gangster T. Annius Milo found in 52, the plea that he killed his political rival Clodius in self-defence, entered on his behalf by Cicero and published as the Pro Milone, failed to prevent his exile. Other Cicero speeches in defence of alleged murderers took a different line. Both Sextus Roscius of Ameria, accused of parricide in 80, and, in 66, Aulus Cluentius Habitus, who was alleged to have killed his stepfather several years earlier, were defended by a combination of outright denial and vilification of the motives of the prosecution team (and, in the Cluentius case, of the victim as well). Cicero's technique was to combine a discussion of the facts with allegations about character and he had little to say on points of law.

Murder was, then as now, both serious and fascinating. Unexpected or unexplained deaths required explanation. Famous, convenient or mysterious deaths generated conspiracy theories. In the absence of forensic or medical evidence, death by poison was more easily alleged than proved – or disproved.