In Act 2, Scene 1 of Julius Caesar, Brutus makes the following statement to himself:
Since Cassius first did whet me against Caesar,
I have not slept.
Between the acting of a dreadful thing
And the first motion, all the interim is
Like a phantasma, or a hideous dream:
The Genius and the mortal instruments
Are then in council; and the state of man,
Like to a little kingdom, suffers then
The nature of an insurrection.
Brutus has made the decision to act as the leader of the assassination plot against his friend Caesar. He feels as if he is now being led by some sort of supernatural force and no longer has control over his own thoughts, feelings, and actions. He does not know exactly how the plans to kill Caesar will be accomplished, but he knows the assassination is now inevitable and that he is only an instrument of fate.
Macbeth is in a comparable state of mind when he makes the decision to murder Duncan and imagines that he sees a dagger leading him towards the King's chamber.
Is this a dagger which I see before me,
The handle toward my hand? Come, let me clutch thee.
I have thee not, and yet I see thee still.
Art thou not, fatal vsiion, sensible
To feeling as to sight? Or art thou but
A dagger of the mind, a false creation
Proceeding from the heat-oppressed brain?
Both Brutus and Macbeth are reluctant to go through with what they have decided to do. Brutus has been persuaded by Cassius, just as Macbeth has been persuaded by his wife. In both men it might be said that their states are suffering the nature of an insurrection, i.e., they are at war with themselves. Brutus would not have acted against Caesar without the goading of Cassius, and Macbeth would not have murdered Duncan had it not been for the relentless goading of his wife. Both Brutus and Macbeth are doing something they really do not want to do. Perhaps both of them sense intuitively that they are setting a train of events in motion that will end in their own destructions. They are acting against their better judgments, allowing themselves to be manipulated by inferior people.
Brutus must be more of a thinker than Macbeth, who is always characterized as a man of action. Macbeth is only concerned with his own thoughts and feelings as he goes to murder Duncan, whereas Brutus speaks in abstractions and generalities. Brutus is taking an almost philosophical interest in his own inner feelings and making general observations about how "the state of man" is affacted by making decisions to undertake "dreadful" things.
It is noteworthy that both men are afflicted with insomnia because of what they are doing. Brutus has not slept since Cassius first broached the idea to him of assassinating Caesar. Macbeth hears a voice crying "Sleep no more!" immediately after killing Duncan. This sleeplessness would account for the hallucinations both men suffer.