Brutus is one of the main figures in Shakespeare's work, Julius Caesar. He is characterized as a staunch supporter of the Roman Republic. This is to say that the wants a government of checks and balances and believes in the importance of the senate. This point is important to keep in mind, because Brutus' ancestor drove out the last king of Rome to establish the Republic. So, the Republic is something that runs deep in the blood of Brutus. So, he opposes any man being dictator, let along dictator for life, even if it is his friend, Caesar.
With all that said, Brutus is also characterized as one who has genuine fondness for Caesar. This affection is what make his act so tragic. Also Brutus is portrayed a man of great honor. His enemies know this and therefore use this to enlist the help of Brutus. From a historic point of view, Brutus is characterized in a similar way to Cato, another man of honor and principle.
Based on the above points, we can say that while others kill Caesar out of envy and rivalry, Brutus does so out of loyal to the state. He is the only conspirator that actually plots to kill Caesar for the sake of Rome. He is sort of a throw back to the early Republic. We can even say that he embodies the virtues of Rome. For this reason he is the tragic hero. He is torn between his love for his friend and the state. He chooses the later, as any good Roman would.