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.