Concerning your question about Shakespeare's Julius Caesar, much goes on in the play and much meaning is revealed, but the enotes Study Guide on the play cites two main themes: regicide and ambition.
The killing of a tyrant, or tyrant to be, dominates the early acts of the play, and drives the action the remainder of the play. Issues and questions are raised by the play: is it just to kill a tyrant, or tyrant to be, to stop his tyranny? under what circumstances is it just? what are the consequences of doing so?
Of course, in Julius Caesar, Shakespeare seems to come down on the side of monarchy, which of course he lived under.
Ambition is seen most notably in the character of Cassius. He sees himself as just as deserving as Caesar to be ruler. He doesn't see Caesar as any better than he is. He manipulates Brutus into joining the conspiracy on the basis of his honor, but he is really lacking in that respect. Brutus soon learns that Cassius is taking bribes, trying to add to his fortune. Antony understands that Brutus only, out of all the conspirators, acted out of honor and concern for Rome. The rest, apparently, acted out of envy and ambition.