Was Brutus of Julius Caesar a villain or a hero?
Brutus is certainly presented by Shakespeare as a tragic hero in Julius Caesar. Indeed, he has all the classic hallmarks of this character type; he's a fundamentally noble, decent individual brought low by a tragic flaw. In the case of Brutus, his flaw is that he's too trusting, too ready to believe—like a lot of virtuous, honorable people—that others are as motivated as he is by the best of intentions. This leaves him vulnerable to the wily overtures of Cassius, who successfully gets Brutus to join the plot against Caesar, playing on his nobility and sense of duty to Rome. Unlike Cassius, whose motives are completely self-interested, Brutus genuinely believes that he's doing the right thing. He has a strong, abiding commitment to the cause of republican liberty, and believes that Caesar's growing political ambitions represent a threat to that ideal.
By participating in Caesar's murder, Brutus is of course betraying a friend; indeed, it's the ultimate act of betrayal. So it's important not to romanticise him too much. He's only human, after all. Our evaluation of Brutus is largely subjective, irrespective of how he's presented in the play. Some may agree with his actions, seeing him as helping to rid his beloved Rome of a would-be tyrant ready to destroy ancient traditions of republican liberty and order. On the other hand, others may choose to look at him as a vain, foolish man who allowed rogues such as Cassius to play on his vanity and embroil him in a wicked murder plot, one that ended in the death of a man who was supposed to be a close friend.
Brutus was certainly not a villain. He had a splendid reputation for integrity in Rome, which was the main reason, at least according to Shakespeare, that Cassius was so anxious to get him to join in the conspiracy. Here is Marc Antony's assessment of Brutus, his enemy, after Brutus committed suicide on the battlefield at the end of the play:
This was the noblest Roman of them all.
All the conspirators save only he
Did that they did in envy of great Caesar.
He only in a general honest thought
And common good to all made one of them.
His life was gentle, and the elements
So mixed in him that nature might stand up
And say to all the world "This was a man."
Whether or not Brutus was a hero is a matter of opinion. It seems to me that both Julius Caesar and Marc Antony displayed more of the conventional heroic qualities. Brutus was primarily a philosopher and did not seem particularly ambitious. There is undoubtedly a great difference between the real Brutus of ancient Rome and the character created by Shakespeare, who depicted him as kind, gentle, thoughtful, and studious.
Brutus says of himself during his quarrel with Cassius in Act 4, Scene 2:
O Cassius, you are yoked with a lamb
That carries anger as the flint bears fire,
Who, much enforced, shows a hasty spark
And straight is cold again.
I don't believe that Brutus can be described either as a villain or a hero.