second Brutus is referred to as "Decius" throughout the play and I will do so too, since that will conveniently prevent confusion between the two Brutuses.
Decius was another one of Caesar's generals during the Gallic conquest. In fact, he commanded the fleet at one point, and after Caesar's victory he served as governor of Gaul for a couple of years. His relationship to Caesar was so close that the Dictator even named Decius as one of his heirs, in case no member of his own family survived him.
... the noble Brutus. ..
Yet despite the importance of the individuals in the conspiracy, the need is felt for something more. Cinna says:
O Cassius, if you could
But win the noble Brutus to our party-
- Act I, scene iii, lines 140-41
Casca explains a little later:
O, he sits high in all the people's hearts;
And that which would appear offense in us,
His countenance, like richest alchemy,
Will change to virtue and to worthiness.
- Act I, scene iii, lines 157-60
There is another reason why Brutus is desired: to cast a respectable cloak over what otherwise might seem a heinous deed.
But Cassius explains his scheme of deluding "noble" Brutus with fake messages and even has them help in distributing them.
... no personal cause.. .
The scene now shifts to Brutus' house. Brutus has been unable to sleep. He wishes to join the conspiracy, but what he needs is some high-sounding noble reason to do so. He can't admit to the world, or even to himself, that he is being driven to it by Cassius' skillful appeal to his own vanity. He says:
/ know no personal cause to spurn at him,
But for the general. He would be crowned.
How that might change his nature, there's the question.
- Act II, scene i, lines 11-13
That seems to be the key to the noble cause he seeks-how power might change Caesar. He decides he will
... think him as a serpent's egg
Which hatched, would as his kind grow mischievous,
And kill him in the shell.
-Act II, scene i, lines 32-34
What Brutus is now thinking of is a kind of preventive assassination. Caesar must be killed not because he is tyrannical but because he may grow tyrannical.
There is appeal in this argument. Power does tend to corrupt, as history has amply proven, and it is tempting to reason that a tyrant is best removed before he has a chance to show that corruption. What if Adolf Hitler had been assassinated in 1932?
And yet, it is a dangerous view. Once we accept the fact that assassination is justified to prevent tyranny rather than to punish it, who would be safe? What ruler could be sure of not being regarded by someone somewhere as being on the high road to tyranny, which he would reach someday?
... Erebus itself ...
Brutus has been receiving the faked letters Cassius has prepared for him and he has managed to talk himself into believing in the nobility of the enterprise. It is clear he intends to join the conspiracy and yet he is still uneasy about it.
When the conspirators arrive at his house, cloaked in masks and darkness, he is aware of the intrinsic shame of conspiracy. He apostrophizes personified conspiracy and says it must assume a false front, for
... thy native semblance on,
Not Erebus itself were dim enough
To hide thee from prevention.
-Act II, scene i, lines 83-85
In some of the more poetic tellings of the Greek myths, Erebus is pictured as the son of Chaos, the brother of Night, and the father of the Fates. There are no tales told of him, however, and in poetry he is merely, as here, used as the personification of darkness. (The word is also used, sometimes, to describe an underground region en route to Hades.)
... what of Cicero.. .
The conspirators are now all together and Brutus is formally accepted among their ranks. Should still others be recruited? Cassius asks:
But what of Cicero? Shall we sound him?
I think he will stand very strong with us.
- Act II, scene i, lines 141-42
Cicero had a very high reputation in Rome in some ways. In an age of general corruption, Cicero was widely recognized as an honest man of high ideals. He was a true republican and favored republican institutions backed by an honest and upright Senate. He would certainly be opposed to Caesar as king. All agree at once, therefore, that Cicero would be an excellent addition.
All but Brutus, that is, for he says:
O name him not! Let us not break with [confide in] him;