Having heard the news of Caesar's assassination, the Roman crowd assembles near the forum in Act III, Scene 2. Crying "Let us be satisfied," the crowd demands explanation for the death of Caesar. Brutus explains that Caesar had become a tyrant, so he and the others slew him. He tells the crowd,
Not that I loved Caesar less, but that I loved Rome
Here, Brutus claims that the death of Caesar is necessary for the benefit of Rome, to keep men free, and that, if the time comes that he is a detriment to his country as Caesar was, he should be put to death likewise:
With this I depart—that, as I slew my best lover for the
good of Rome, I have the same dagger for myself, when it
shall please my country to need my death. (3.2.46-47)
Ironically, then, a plebeian cries out, "Let him be Caesar" and another says that Caesar's attributes will be crowned in Brutus if he is made ruler.
The contrast between what the plebeians mean and what actually happens is the fact that the assassination by Brutus and the others initiates a civil war which is worse that any battle that Caesar has waged as it pits Roman against Roman. And, Brutus does, indeed, become a detriment as Caesar did to his country--he "becomes Caesar"--and, later defeated, dies by the sword himself. In fact, Brutus and Cassius both die for the betterment of Rome as with their deaths the civil war ends.