Brutus is driven by honor and loyalty, yet he fails to realize that these noble qualities have blinded him from the severity of his actions, as well as the consequences that came with them.
By killing Caesar, Brutus and the conspirators ironically transformed him into a martyr for the people of Rome, thus magnifying his power and undermining the sanctity of the great Republic they were trying to save.
The conspirators soon realize that their call for justice was short lived, and one by one they commit suicide.
The Rome that Brutus and Cassius fought for was destroyed by their very actions, as Antony and Octavius eventually rise up to seize absolute power and declare the Roman Republic obsolete.
With Caesar gone, the glorious Republic became unstable and fell, ironically at the ends of the men who attempted to save it.
Perhaps the most tragic flaw of mankind is its ability to build up glorious civilizations, and then tear them down when the results of their efforts do not satisfy the needs of a few.
Mark Antony rises up during the chaos following Caesar's death, and uses it to his advantage by depicting how treacherous the murder of Caesar was. Antony sees how the conspirators have become blinded by their loyalties to the Republic, and that with Caesar's death, the Republic has now been tainted forever.
Antony eloquently states that since the conspirators have killed one of the "noblest men that ever lived in the tide of our times" (III. 77-78.) he will wreck havoc and promises to "let slip the dogs of war" (III. Antony then uses his words to bring the Republic, which the conspirators killed for, against them.
Brutus' greatest loyalty is to the Republic, and in his mind no man should have the right to tamper with the stability of Rome.
Without Rome, Brutus would be a shell of a man, and he fears that Caesar's rise to power would destroy the ideals he holds true, and in turn destroy Brutus as well.