Would Brutus, from the Tragedy of Julius Caesar, be considered as a villain or a hero?

Although Brutus does play a pivotal role in Caesar's assassination, he is not really considered to be a villain because his motivations are not selfish or full of malice. Instead, Brutus is considered to be one of classic literature's tragic heroes.

Brutus is a well respected and he is a genuinely honorable man. He is honest, and wants only the best for Rome. While he is close friends with Julius Caesar, he is alarmed by Caesar's behavior and Brutus is afraid that his good friend has become corrupt and will be soon ruling Rome as a Dictator. As a result of these concerns, Brutus becomes involved in the plot to assassinate Caesar, believing that Caesar's death will benefit Rome. It is not that he wants his good friend dead. It is that he wants Rome to be saved from a Dictator and a tyrant.

Unfortunately, the men with whom he collaborates are not as honorable or as honest as Brutus. He does not realize this. This is apparent in Act II, Scene I where Brutus refuses to take an oath of loyalty to his fellow conspirators. He refuses because he feels that if everyone involved is truly honest, they should all be able to see this on one another's faces and that an oath is not necessary. This exemplifies his naiveté as a politician, and even as a man. He is betrayed later by some of these conspirators, much to his surprise. 

Brutus is a tragic hero because he is brought down by his own tragic flaws: naiveté and idealism.