Have you ever known someone to be chosen for a job or elected for an office who almost instantly became engulfed with the power of the title? It happens all the time with humans...Caesar is no different. It is human vanity that allows us to become self-important and all puffed up with ourselves.
Brutus is more loyal to Rome and the ideals of Rome than to any one person. This is his weakness, and the other conspirators knew this. They left him notes and questioned him until he began to question himself. He could not allow Rome to become anything other than the honorably governed institution he has come to love and serve. He agrees to rise up against Caesar to preserve Rome.
Caesar thinks that he cannot be killed, that he is in fact immortal. He acts nonchalantly towards the soothsayer when he warns caesar of the ides of march. Also Caesar fails to see the Omens and Signs that are cleary pasted in front of his face. When his wife warns him of the dream she has of the conspiracy he laughs at her and allows Brutus to take him to the conspirators.
Caesars main source of blindness is in fact his arrogance, pride and fate. Strangely enough in the beginning of this book caesar acts very superstituos, telling mark antony to touch Calpurnia when he is running so she might become pregnant. But when the soothsayer tries to warn him all trace of superstitous behavior evaporates.
Brutus was a loyal servant of Ceasar's. What made him change his mind to the extent that he was contemplatig murder and even executed what was on his mind?