If anything destroyed the idea of republican Rome, it was the concentration of power into the hands of one man. And this man was always a general. I mean, you can't march on Rome without an army, after all. Why were there such powerful generals? Because Rome had decided to become an empire and empires need to expand militarily. Particularly the Roman Empire needed to expand militarily because it always needed new land to give its retired legionnaires. That expansion created the all-powerful general and the incorporation of diverse peoples made it easier for them to be loyal to him rather than to some abstract idea of the republic.