Which reason does Brutus give to justify killing Caesar?

Brutus justifies killing Caesar on the grounds that Caesar would become a king. In his soliloquy in the opening scene of Act II, Brutus describes his former friend as a "serpent's egg/ Which hatch'd would as his kind grow mischievous." Brutus says in the same soliloquy that he has no "personal cause" to kill Caesar. After the deed is done, Brutus observes, "ambition's debt is paid," a theme he repeats in his speech to the people of Rome. He says Caesar was his friend, but that "as he was ambitious, I slew him." Essentially, Brutus fears Caesar will, by virtue of his exploits and popularity, name himself king of Rome, overthrowing the Republic Brutus feels honor-bound to protect. Caesar was Brutus's close friend, but he was a threat to Rome and Roman liberties, so Brutus killed him. Rome, he assures the crowd, comes before his personal feelings. Some of the assassins had ulterior motives, but Brutus acted from a sense of patriotic duty.