What is the moral lesson from Julius Caesar?

The lesson from this play is that arrogance can have deadly results.

Julius Caesar died because he was arrogant.  Arrogance is about more than having a high opinion of yourself.  It means that you put your judgement above everyone else’s.

Cassius and Brutus killed Caesar because he was arrogant, and because he was ambitious.  Caesar’s arrogance led to the ambition.  He did not care what anyone thought of him.  Caesar was Caesar.  For example, he ignored all of the warnings that his life was in danger.  Caesar knew better.

You can tell Caesar was arrogant by his reaction to the conspirators’ suit over Metellus Cimber’s brother.  Caesar should have realized that the men were up to something.  They surrounded him, pleading with him, and he thought nothing of it.  His reaction is supremely egotistical. 

I could be well moved, if I were as you:
If I could pray to move, prayers would move me:
But I am constant as the northern star,
Of whose true-fix'd and resting quality
There is no fellow in the firmament. (Act 3, Scene 1)

The conspirators are able to kill Caesar, but then they are in a bind.  They are the second example of the lesson that arrogance is deadly.  Brutus and Cassius were just as arrogant as Caesar.  They did not want Caesar to be dictator, so they assumed that they knew better than anyone else.  What gave them the right to kill the leader of Rome?

Brutus and Cassius, and the other conspirators, paid for their arrogance.  Brutus believed that the movement needed to avoid killing Mark Antony because he wanted to keep the assassination clean.  He did not want to be considered a butcher.  Brutus put principle over common sense. 

We shall be call'd purgers, not murderers.
And for Mark Antony, think not of him;
For he can do no more than Caesar's arm
When Caesar's head is off. (Act 2, Scene 1)

Not killing Antony was a mistake.  Brutus assumed that Antony would not be any trouble.  He even agreed to let him speak at the funeral.  Antony was ambitious and aggressive too though.  He desired revenge for Caesar’s death.

Antony was able to swing the Roman people over to his side with his excellent funeral speech.  He left Brutus in the dust.  Brutus and Cassius both eventually ended up dead, because again Brutus acted arrogantly during the battles against Antony and Octavius’s armies.  Antony would pay for his arrogance later.