are you referring to the Late Roman Republic?

Ok, this is a huge and complicated topic and I'll do my best. The problem with the Republic was it was a government designed to run a city-state but now being used to run an empire from Spain to the Syrian coast. The competition for offices had increased to astronomical levels and provincial postings offered the opportunity to gain immense levels of wealth. This meant fight for the limited number of offices meant bribery and holding extravagant spectacles became increasingly prevalent which only increased the competition for offices so as to get a provincial posting to generate enough money to pay off all your debts incurred getting elected (c.f. Verres).

The violence was interjected with the Gracchi, two tribunes of the plebs who felt the system was breaking down (too many of the independent freeholders that Rome depended on for soldiers were now landless and their land worked by the cheap slaves (through all the conquests) on senatorial megafarms. So they sought to pass legislation that would force senators to abide by an older law restricting the amount of land one could possess - and they ignored the senate when it did not endorse the proposed law. Traditionally senate endorsement was always sought before a bill was submitted. For a variety of factors, the senate feared the Gracchi's growing clout and support amongst the plebs and both were sequentially murdered - one by a group of senators and the other by mercenaries kept by another senator following a war. No one was punished for these two murders and it introduced political violence as another option.

So with the ultra-competitive jousting for offices in an incredibly high-stakes political arena (where losing an election could mean bankruptcy), resorting to violence through political gangs or veterans or clients as a way of intimidating your political opponents was adopted by many factions.