Never forget the Gracchi brothers. Populist reformers who were both executed by the Senate because of the threat they represented to patrician power. The older of the two was killed in an area of Rome where weapons were not permitted, by senators, while he was Tribune and technically sacrosanct.
A word on the Gracchis. The Gracchis were radical reformers by the standards of their time - they wanted to take lands from large landowners and give it back to landless peasants. Tiberius Gracchus was clubbed to death in broad daylight. Gaius committed suicide when he knew his death was inevitable.
What's interesting is that when studying the fall of the Republic, it makes sense to start with these guys. Roman politics had always been high stakes and cut throat, but this was when it took a turn for the violent. Starting 130BC, it started going downhill. In the next few decades, this violence gradually became regular civil war, especially after military reforms that meant that poor people could sign up to be soldiers.
By around 60BC, a politician would run up massive debts to get elected, try to make it as much money during the 1 year consulship and then try to rob whichever province they became governor of. If they lost the election, they had no hope of paying back the debts, unless they launched a civil war, overthrew the state and seized power. Catiline was one such person.
Things got worse after that. The sham of democracy remained, even after Augustus cemented power.
Point being, the Republic took nearly a century to die, it didn't happen overnight. The degrading of politics was a gradual process. Previously unthinkable acts are normalised. Good people who are disgusted with the state of politics leave, and the horrible people remain, leading a race to the bottom.
When politics goes bad, it goes really bad.