Tiberius Gracchus's murder introduced political killing as an acceptable tool

In 133 BCE, the tribune Tiberius Gracchus proposed redistributing public land to landless Roman citizens, a policy aimed at reversing decades of wealthy landowners absorbing small farms and displacing the peasant families who had traditionally supplied Rome's legions. The Senate's conservative faction, alarmed at the threat to its members' land holdings and at Gracchus's willingness to bypass normal procedural channels to pass his bill, ultimately responded not with counter-legislation but with a mob of senators who beat Gracchus to death in the street.

Duncan treats this killing as the true opening act of the Republic's long collapse, since it was the first time in generations that a domestic political dispute had been resolved through direct violence against a sitting magistrate rather than through the Senate's traditional, if messy, procedural maneuvering. Once this line had been crossed and its perpetrators faced no real punishment, it became a precedent available to future generations of Roman politicians on both sides of any dispute.

Takeaway: the first time a political rival is killed rather than out-argued, a threshold has been crossed that is nearly impossible to uncross.