One man’s hero is another man’s villain.

 "Many people in the Renaissance were passionately interested in the story of Caesar's death at the hands of his friends and fellow politicians. There was much debate about who were the villains and who were the heroes. According to the fourteenth-century Italian poet Dante, Brutus and Cassius, the foremost of the conspirators who killed Caesar, were traitors who deserved an eternity in hell. But, in the view of Shakespeare's contemporary Sir Philip Sydney, Caesar was a rebel threatening Rome, and Brutus was the wisest of senators. Shakespeare's dramatization of Caesar's assassination and its aftermath has kept this debate alive among generations of readers and playgoers."–Mowat, Barbara, and Paul Werstine

Both Caesar and Brutus are perceived to be heroes and villains in Julius Caesar At the opening of the play, Caesar is hailed for his conquests and is admired for his apparent humility upon refusing the crown. However, once murdered, Caesar is painted (by Brutus ) as a power hungry leader with the intentions of enslaving all of Rome. Brutus' speech, which follows Caesar's death, successfully manipulates the plebeian perspective. By the end of his speech, the crowd is hailing Brutus for killing Caesar, whom they now perceive as a great villain. But, the crowd is easily swayed once again when Antony speaks. Following Brutus' remarks, Antony gives Caesar's eulogy, manipulating the crowd with stories of Caesar's kindness, and sharing the details of Caesar's will, which leaves money to every Roman. At the end of Antony's speech, the crowd is once again supportive of Caesar, mourns his death, and seeks to kill Brutus, Cassius, and the other murderers. The swaying opinions of the plebeians, and the great differences in opinion that the play presents leave the audience to determine who, if anyone, is the hero of the play, and who, if anyone, is the villain. 

In Shakespeare's Julius Caesar Marcus Brutus is the most complex character to analyze, but he is also the tragic hero. Although Brutus was greatly involved in Caesar's murder, it doesn't make him a villain. In his soliloquies, the audience learns of the motives which drove Brutus to action. He is a powerful leader, a compassionate husband, and also a true and trusting friend. He does not kill Caesar because of envy, greed or for the good of himself, but he explains that he assassinated him "not that I loved Caesar less, but that I loved Rome more" ( III.ii.21-22). With these words Marc Antony and the public realize he acted from civic duty to Rome and for its future. In Brutus' mind his actions were the right choice to make. This makes Brutus the tragic hero in the play.

 Those who see Brutus as the villain tend to fault Brutus for being gullible and so easily manipulated.  He falls for Cassius' ploy to kill Caesar unable to see the personal vendetta attached to Cassius' efforts.

Others fault Brutus for his lack of loyalty.  Caesar had been one of his closest friends for a long time.  Some people can't justify stabbing one's friend in the back, no matter what the reason.  They are further repulsed by the way Brutus bathes his hands in Caesar's blood almost immediately after the slaying.  They find it a gory act, void of any loyalty or honor. 

 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.

Cassius is the ringleader of the conspirators. He's politically savvy and manipulative, and he absolutely resents the way the Roman people treat Julius Caesar like a rock star. More important, he hates the way Caesar runs around acting like a god: "Why, man, he doth bestride the narrow world /Like a Colossus, and we petty men / Walk under his huge legs and peep about" (1.2.142-144).

Cassius is also responsible for manipulating Brutus into joining the conspiracy (although Brutus may have already been thinking of turning against Caesar):

Well, Brutus, thou art noble. Yet I see
Thy honorable metal may be wrought
From that it is disposed. Therefore it is meet
That noble minds keep ever with their likes;
For who so firm that cannot be seduced? (1.2.305-309) 

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."