Greed and corruption has been around for as long as land and money have.

There were substantial conflicts between rich and poor Romans well before the end of the Republic. It was not uncommon for a lower class Roman to go off and fight with the army only to come home and find some rich fuck effectively squatting on his land.

This was turning into something of a large scale crisis 60+ years before Julius Caesar crossed the Rubicon. At the same time, politics was dominated by the wealthy and/or those who were successful military leaders.

The Republic didn't really have "checks and balsnces" in some cases so much as "social conventions". More a common understanding that something is just not done. They were actually rather Ill equipped to deal with individuals who had a thirst for power. Tiberius Gracchus, who served as Tribune, gave away lots of state owned land to some of the poorer Romans. He did so without consulting the Senate which raised a lot of eyebrows. When he attempted to stand for a second term as Tribune in 133 B.C. -- which was just "not done" -- the Senate responded by murdering him.

Tiberius' brother, was elected as Tribune. He went further than Tiberius and sponsored a whole bunch of legislation which would have benefited poorer Romans. The Senate responded by murdering him as well. Over the next few decades, there were a handful of successful military leaders who clamored for more and more power. At the same time, the public was growing increasingly dissatisfied with the Republican government.

Julius Caesar was smart enough to recognize that the Republic was becoming frail and ballsy enough to give it a good shove down the stairs. In 82 B.C. he marched his legion into Rome -- a clear act of treason -- and effectively declared himself dictator. He was met with little resistance. He was viewed as a champion of the poor in some ways and, based on the way the Republic treated poor Romans, they were probably looking for a champion.

There were others who wanted to wrest Caesar's newly acquired power from him. Pompey, another successful Roman general, went to war with Caesar. Pompey had the support of the wealthy Aristocrats. Pompey lost.

The Senate murdered Caesar in 44 B.C. Caesar's supporters responded by killing his assassins. Then they turned on each other. That marked the beginning of another civil war from which Octavian, Julius' nephew and adopted son, ultimately emerged victorious. By then, the Republic was effectively dead.

To make a long story short, Julius Caesar didn't break the system. It was already broken. He managed to exploit it further than than anyone else had up until then but there were glaring cracks in the foundation of the Republic that directly contributed to it's demise. The imbalance of power between the elite and the poor was definitely a big "crack".