1. Rome was not a democracy. It was a republic. The fall of the republic started before 100bc.
2. The fight between Sulla and Marius accelerated the fall, as did the corruption ofthe Senatorial ranks.
3. Caesar was a brilliant political and military mind. He had no constituency among the nobles so he championed the cause of the plebs.
4. Do not forget that Pompey, Crass us and Caesar.
You can probably date the fall of the Republic from any of these spots.
In addition, the Roman Republic had dozens of short-term dictators. Most of these were elected to break deadlocks in the Senate or oversee elections, but some were used to restore order or wage war. The move from this to Julius Caesar and on to empire wasn’t such a great jump.
Seems to me it began with Tiberius Gracchus using violence in the Senate. His cause was, I think a good one, his methods established the precedent that would eventually destroy the Republic as if Senators and Representatives are openly beaten and killed in the chambers or such.