There is an age old question. Do leaders and individuals shape history? When it comes to the Roman Republic, we can answer in the affirmative. The late Republic has so many towering figures that it is nearly impossible to see Rome apart from these figures and their actions. In light of this, we can say in the least that the towering figures of the Republic did contribute to the fall of the Republic for a few reasons.
First, powerful general created civil wars and severely damaged the constitution of Rome. This was possible, because armies were often more loyal to their generals than to the state. So, these larger than life figures led Rome into civil war. There was a civil war between Marius and Sulla in 88-87 B.C. There was almost another civil war between Catiline and the Roman state, which Cicero stopped. Then there was the massive war between Caesar and Pompey. By these actions, the fabric of Rome changed.
Second, there was also manipulating of the constitution and using religion for personal gain. We see this is the gang fights between Milo and Clodius. In fact, violence in Republican Rome grew to fever pitch. Also we see this in the area of religion, where religion was used (for example Bibulus), to gain political power and make invalid the work of others.
In conclusion, if any period of history can be seen through the influence of personalities, it is the Republic.