What is it that the audience learned from Brutus's soliloquy beginning "It must be by his death..." in Julius Caesar?
We learn that Caesar has not actually done anything wrong, according to Brutus, but is considered too dangerous to be kept alive because of what he might do.
In this soliloquy, Brutus is referring to the need to kill Caesar before he becomes too dangerous. Brutus is essentially trying to talk himself into killing Caesar. He has agreed to join the conspiracy to assassinate him, but it is not because Brutus is an ambitious man who wants to be king. He is trying to prevent anyone from becoming king of Rome. He wants control of Rome to rest with the senate, and specifically his allies.
Brutus tells us in this soliloquy that he personally has nothing against Caesar, but that he is opposed to him on principle because he is too ambitious.
It must be by his death: and for my part,
I know no personal cause to spurn at him,
But for the general. He would be crown'd:
How that might change his nature, there's the question.
It is the bright day that brings forth the adder;
And that craves wary walking. (Act 2, Scene 1)
He continually compares Caesar to a baby serpent. While still in the shell, he is not dangerous. In other words, as long as Caesar is not crowned king, he is not to be feared. However, as soon as he has that power, he will be. This is why Brutus says that if Caesar were crowned it “might change his nature” and make him into the dangerous snake they all know he can be.
Even though Brutus tells us that Caesar has not done anything yet that, there are good reasons for suspecting Caesar. Brutus is quite clear. Caesar is ambitious. He explains why.
The abuse of greatness is, when it disjoins
Remorse from power: and, to speak truth of Caesar,
I have not known when his affections sway'd
More than his reason. But 'tis a common proof,
That lowliness is young ambition's ladder … (Act 2, Scene 1)
In other words, Caesar is a self-made man and because of this, he is more likely to abuse his power. He has had a taste, and will now become more and more power-hungry until he is unstoppable. He will “grow mischievous.” Brutus feels that this is inevitable, even though he has seen no direct evidence of it yet.
Since Caesar is so dangerous, even though his power is in its infancy, Brutus decides that he must join the conspirators in killing Caesar. This is a remarkable judgement to come to, considering the circumstances. Rome is supposed to be a republic, with elected leaders. Brutus also is very close to Caesar, loving him like a father. He knows Caesar well. If anyone can predict how he may act, it might be Brutus. Brutus decides then, that the only way to handle Caesar is to “kill him in his shell.”
This soliloquy is really an exploration of Brutus’s mind and motives. In explaining why he is about to commit murder, Brutus never demonstrates hints of ambition himself. He considers himself a liberator of the republic. He is freeing the people of Rome from the tyranny of their leader. Although he is not always inclined to act on his own, Brutus has been pressed into service by Cassius. Now that he has come to this decision, he will take the lead. If Brutus is going to lend the dignity of his family name to this operation, he wants to be in charge.