Why did Brutus betray his best friend Julius Caesar?
In his speech to the Romans after Caesar has been slain, Brutus explains that he loved Caesar, but " . . . I loved Rome more" (3.2.22). After Caesar returns to Rome, having slain Pompey with whom he was in the First Triumvirate, Brutus wonders if Caesar has become too power hungry and if he would be honorable and just if he were granted the tremendous authority of ruler of Rome.
When Cassius talks with Brutus in scene 2 of act 1, Cassius suggests Caesar's tyranny, describing Caesar as now having "become a god" (1.2.122). Further, he describes Caesar as being "like a Colossus, and we petty men/Walk under his huge legs and peep about" (1.2.142-143). Then, in the early hours of the next morning, Brutus walks in his orchard because he cannot sleep. He ponders what Cassius has said, and he also reads a letter, forged by Cassius, that has been thrown through his window during the night. This letter accuses Brutus of not being aware of the threats to Rome: “Brutus, thou sleep'st. Awake, and see thyself” (2.1.46). Brutus interprets this letter as a protest by others against Caesar: “Thus must I piece it out: / Shall Rome stand under one man's awe . . . " (2.1.51-52) Finally, after much thought, Brutus decides that Caesar is like "a serpent's egg" which in time will become dangerous. It is then that Brutus decides Caesar must not be allowed to become the sole ruler of Rome. He still loves Caesar, but in his flawed idealism, Brutus feels he must act for the greater good and protect the Roman state and its people.
Brutus knew Caesar was changing. Power corrupts. Antony had offered Caesar the crown. This indicates that Antony desired to see him as king. Even though Caesar refused it, Brutus began sensing that Caesar was entertaining the idea. If he had not been, he would not have allowed Antony to offer him the crown three times.
Brutus sensed that the freedom of Rome was at stake. Truly, the two were close friends. That is evident in Caesar's dying words. As the conspirators began stabbing Caesar, Brutus too pulls out his dagger. To this action, Caesar cries, "Et tu Brute? Then fall Caesar." This means "You too Brutus? Then fall Caesar."
These last words indicate that Caesar may not have known just how ambitious he was becoming until he saw Brutus with his dagger in hand. According to Caesar's last words, he had so much confidence in Brutus until he knew he must die for his ambition. In other words, if Brutus was in on the conspiracy, Caesar realized there must be a dangerous flaw in his character, and he agreed to fall.
Caesar's last words reveal the good that had been in Caesar. He had total faith in Brutus. Brutus would never have been in on the conspiracy had he not realized Antony was intent on crowning Caesar. The mistake Brutus made is in allowing Antony to live. Brutus is not the type man to make rash decisions, and Caesar knew that. Because of Antony, Brutus dies on his own sword.