Let's be clear. You have to differentiate between history and Shakespeare. Shakespeare was not interested in writing history lessons.
And one of the brilliant things about this play is that Shakespeare never answers your question. Brutus agonises in his soliloquy in Act 2, Scene 1, about what he has do to, concluding (at the start, strangely, of his soliloquy) that "it must by his death". Brutus is best of the conspirators at making it clear to the conspirators that what they are doing is a service to Rome, is noble, is worthy, is morally right - and so on:
Let us be sacrificers, but not butchers, Caius.
We all stand up against the spirit of Caesar,
And in the spirit of men there is no blood.
O, that we then could come by Caesar's spirit,
And not dismember Caesar!
He's a persuasive speaker. And yet, you have to ask yourself whether he's right. Caesar kills Flavius and Marullus for "pulling scarves off Caesar's images". But other than that, is there any evidence of tyranny? No. He's a weak, infirm, arrogant, epileptic old man who seems to quite fancy being king. Does he deserve murder, as Shakespeare puts it? Hard to say. Can you ever say?
What you can say, however, is that Brutus (as he does at every stage in the play) gets it wrong in the above quote. There is certainly blood. Lots of it. And then the conspirators have to bathe their arms in it, and then try and claim that it isn't a "savage spectacle". It is.
None of that really helps us answer the big question though. Did they do the right thing? Well, how do you evaluate it? On what Caesar was like before? Or on what comes afterwards? Critics and scholars have disagreed and disagreed about the slant of the play. Revolutionaries throughout history have applauded Brutus as a hero. Dante put him and Cassius in the 7th circle of hell. Jury is out on that one. What do you think?
One perspective of whether Brutus and Cassius served Rome by assassinating Caesar points to the successors, the second triumvirate of Octavius, Marc Antony, and Lepidus, as more tyrannical than Caesar himself was. After Octavius defeated Marc Antony and Lepidus was forced to retire, Ocatvius became Augustus Caesar. To the extent that Augustus did not abuse his powers, he was a good leader, but his consolidation of not only military power, but also the tribunical and proconsular set the stage for the end of popular freedom. The republican government fell, giving rise to a single, powerful emperor.
While it is not known what old Julius would have done, as stated above, Caesar was a skilled politician. He began a platform of reform:
- He granted citizenship to many colonials to widen his "base."
- He granted pay to Proconsuls to remove corruption and gain allegiance from them
- He established a network of spies
- He instituted a policy of land reform designed to take power from the wealthy
- He reduced the powers of the Senate so as to make it an advisory council only
Yet, knowing how "the evil that men do lives after them, [and] the good is oft interred with their bones" (III,ii, 76-77) it is, indeed, difficult to pronounce judgment on the act of Brutus and Cassius. However, one thing is certain: the lean, hungry Cassius was clearly serving his own designs. On the other hand, Brutus, politically naive, but--"the noblest Roman of them all"--did act according to his conscience. Even Marc Antony pays tribute to the integrity of Brutus:
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 (V,v,68-73).