My favorite line from Julius Caesar is 'death for his ambition.' That sums up the play perfectly for me.
Caesar was arguably one of the most successful generals and politicians in human history. Caesar was so well loved and so successful that his popularity threatened to tear the Republic apart, by crowning Caesar king of the entire Roman Empire. There was a great fear that he would become a tyrant. For his ambition, Caesar was stabbed to death by his closest colleagues and friends.
Similarly, Bruno became involved in Caesar's assassination for the good of Rome, to save the Republican form of government from being replaced with a dictatorship. For his ambition, Bruno was murdered during the ensuing civil war and everyone thinks of him as one of history's most notorious traitors.
All of the main characters in this play are powerful men, flawed politicians with personal agendas. The main characters frequently voice disdain for the general public, who are fickle and easily manipulated by lofty speeches. The public are described like this:
(reacting to a speech) "the rabblement hooted, and clapped their chopped hands, and threw up their sweaty night-caps, and uttered such a deal of stinking breath..."
"You blocks, you stones, you worse than senseless things!"
Shakespeare lived in a highly political time full of constant King of Thrones type coups and spies and civil wars, and I see this play as his condemnation of that type of world. Power corrupts. People have different takes on this play, but that's mine. You can interpret Shakespeare's plays in about a million different ways, which is a big reason why they keep getting read all these hundreds of years later.
My favorite parts of the play are the ones where supernatural power is invoked, aka 'beware the Ides of March' etc:
"And there were drawn upon a heap a hundred ghastly women, transformed with their fear, who swore they saw men all in fire walk up and down the streets. And yesterday, the bird of night did sit even at noon-day upon the market-place, hooting and shrieking."
No matter how high your ambition soars or how much you achieve, death is always the final result. The power of mankind is nothing compared to the power of the universe. The great wheel turns.