Was the killing of Julius Caesar moral or not moral?

This is a major debate throughout the play. Brutus fears that Caesar will become king, which would destroy the republic of Rome and turn it into a monarchy. It is a possibility: Mark Antony offers Caesar the crown three times in front of a cheering crowd, and Casca claims that the people would praise Caesar even if he “had stabbed their mothers.”

Brutus spends much of Julius Caesar convincing himself to kill his friend. He worries about whether power would corrupt Caesar: “He would be crown'd: / How that might change his nature, there's the question.” Perhaps making him king would “put a sting in him.” Brutus decides that killing Caesar is not personal: “Let us be sacrificers, but not butchers ... / We all stand up against the spirit of Caesar.”

At Caesar’s funeral, Brutus and Antony give speeches that support and condemn Caesar’s murder. Brutus announces that he cared for Caesar: “Not that I loved Caesar less, but that I loved Rome more.” He killed him for the Roman people, not for himself. (The other conspirators do not seem to have such noble motives.)

Antony argues that Caesar was a dear friend, to him and the people of Rome. Caesar refused the crown the three times it was offered, a sign that he wasn’t as ambitious as Brutus claimed. Caesar also donated money to every Roman citizen in his will, again demonstrating that maybe he was a more salutary than dangerous to the Roman people.

Even if Caesar did aim for the throne, there is also the question as to whether it is moral to conspire to murder one’s friend for political purposes. Antony emphasizes Brutus’s personal betrayal of Caesar:

For Brutus, as you know, was Caesar's angel:
Judge, O you gods, how dearly Caesar loved him!
This was the most unkindest cut of all…

Unfortunately for Brutus, Caesar is not simply an idea. He is a man. When the conspirators stabbed him, Caesar had not yet taken the throne or been corrupted by it. Brutus is tormented by guilt, and Rome is eventually ruled by a series of emperors. Perhaps he was on the right side of history by aiming for a more equal society, but Brutus as well as the republic of Rome could not live with the decision that Brutus made.