Caesar was definitely the main problem, they just killed him too late.

Caesar used the genocide in Gaul to shape his troops into an army loyal to him personally. Caesar used his position as pope^1 to create a religious cult that could accept him as god-emperor because Romans knew not to accept kings. Caesar used his position as public tribune to sabotage and reshape the functioning of government. Caesar became imperator and used his authority to dismantle the republic's checks and balances.

Caesar was a unique concentration of power without whose deliberate and seemingly self-guided scheming the existence of a monarch of Rome would have been much harder to swallow. The people would still have been angry and demanded radical change, and maybe they could have found something better. Or perhaps Rome would have weakened in crisis upon crisis and its industrial enslavement of tens (if not hundreds) of millions of people would have come to a much sooner end.

^(1): head of religion in Rome, Pontifex Maximus.