Brutus is portrayed throughout the play as a noble man. Alone among the conspirators, he acted out of concern for the Republic, not out of ambition and jealousy. This is the point that Marc Antony makes at the end of the play, after Brutus has committed suicide in defeat:
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 mix'd in him that Nature might stand up
And say to all the world, “This was a man!”
Obviously, Antony's speech portrays Brutus in the most sympathetic light possible. But throughout the play, Brutus is seen as honorable and motivated by a sense of duty to kill his friend. (As an interesting sidenote, many Shakespearean scholars have argued that Brutus is actually the protagonist of the play.) He agonizes over whether to kill Caesar, particularly in his soliloquy at the beginning of Act II. After the deed, he orders the other conspirators not to kill Antony, saying that they should not be "butchers" and that they should remember the reason for which they are killing Caesar. In short, Brutus is an honorable character, so long as one views honor and duty as more important than friendship.
Throughout the play, Shakespeare portrays Brutus as a noble, morally upright man who truly wants what is best for Rome. Shakespeare influences the audience to sympathize with Brutus by juxtaposing his character with Cassius's. Cassius is depicted as a self-serving, ambitious senator who is jealous of Caesar. He goes to extreme lengths to convince Brutus that Caesar will eventually ruin Rome. Shakespeare also creates sympathy for Brutus by depicting his inner thoughts as he contemplates his role in the conspiracy. In Act Two, Scene 1, Brutus reveals his honorable intentions. Brutus says,
"It must be by his death, and for my part I know no personal cause to spurn at him but for the general. He would be crown'd. How that might change his nature, there’s the question" (Shakespeare, 2.1.10-14).
Brutus goes on to compare Caesar to a serpent in an egg that will eventually hatch. Brutus's intentions depict his selfless nature: he wishes to protect Rome from a potential tyrant. He gives much thought into joining the other conspirators and finds it difficult to harm his good friend.