Idealism as a political liability
Brutus is the most principled character in the play, and his principles get him killed. He refuses to kill Antony, insists on fighting at Philippi against Cassius's advice, and trusts that the crowd will respond to logic. Every idealistic choice makes the conspiracy weaker.
Rhetoric and the power of language
The play shows that how you say something matters more than what you say. Antony's speech is technically less honest than Brutus's, but it works because it targets feeling rather than reason. Language shapes reality in Rome.
Fate versus free will
Omens, prophecies, and Caesar's ghost suggest that the outcome is fixed. But the characters keep making choices that accelerate their own destruction. The play does not resolve whether fate or bad judgment is to blame.
Loyalty and betrayal
Every major relationship in the play involves a betrayal. Brutus betrays Caesar. Cassius betrays Brutus's trust by manipulating him. Antony appears loyal to the conspirators briefly, then destroys them. Loyalty is constantly performed and constantly broken.
The danger of unchecked ambition
Both Caesar and the conspirators are driven by ambition, though they describe it differently. Caesar wants power. The conspirators want to be the men who saved Rome. The play shows that ambition dressed up as virtue is still ambition.