In Julius Caesar by William Shakespeare, Brutus must be labeled an assassin because he was part of the group known as the conspirators who did, indeed, murder Caesar. The difference between Brutus and the others, of course, is that he committed this distasteful act in an effort to save his country from being turned into a monarchy.
Brutus does not do this lightly or selfishly, like the others do. He is not jealous of Caesar, nor does he have any valid fears that Caesar is trying to usurp power and become king. Unfortunately, what Brutus does not know is that all the evidence he is presented with concerning Caesar is fabricated in an attempt to win Brutus over to the conspirators' side. If he had known, he would not have participated in this heinous act.
Once the deed is done, all of the murderers stand boldly over the body, proclaiming their commitment to the people and claiming they assassinated Caesar to protect them from a tyrant. Brutus gives an effective speech, but Mark Antony gives a masterful oration in which he reminds the people of all the good things Caesar has done for the poor and reminds him that Caesar turned down a kingship not once but three times.
Things really turn ugly when Antony's speech ends and he reads Julius Caesar's will which grants some money to every citizen. This plays on the emotions of the people (as he knew it would), and the people suddenly turn on their leaders-turned-assassins. Brutus really has no choice but to flee with the others. Civil war has broken out, the consequence of regicide, and the country is in turmoil.
Brutus really is not the "bad guy" here, though he is guilty of letting himself be deceived into acting against his better judgment. In Act IV scene ii, Brutus does wish he could undo some of the things he has done. He tells Cassius's servant:
Your master, Pindarus,In his own change or by ill officersHath given me some worthy cause to wishThings done, undone.
If Brutus realizes the enormity of his actions, it is in Act IV scene ii.Countrymen,My heart doth joy that yet in all my lifeI found no man but he was true to me.I shall have glory by this losing dayMore than Octavius and Mark AntonyBy this vile conquest shall attain unto.