Rome's flaw was giving power to Cassius who acts out of self interest.
Cassius manipulates Brutus into thinking Caesar is not worthy to be king. Cassius says "Brutus and Caesar" what should be in that "Caesar"? (Julius Caesar, 1.2.147-148). He does this because he is jealous of Caesars power.
"I was born free as Caesar. So were you" (1.2.98-99).
"You lay it in the praetor's chair, where Brutus may find it" (1.3.145-147)
To further his plan, Cassius gets Casca to lay fake letters in Brutus room supposedly from commoners that are praising Brutus saying how he should be king.