According to some roman historians, such as sallust, which moment in republican history marked the beginning of the end of the roman republic?

Sallust, who was a Roman historian and active in politics saw the decline of the Roman Republic through civic corruption, rivalry, and strife among the politicians of Rome. For him, this culminated in the assassinations of Julius Ceasar and Cato, who he believed to have pure intentions and civic duty for the Roman Republic. 

The answer would be a