Macbeth begins to have doubts about the plan to assassinate Duncan. This is one of the first moments when we see Macbeth’s inner conflict. He is compelled by ambition and Lady Macbeth’s influence to carry out the “bloody instructions,” but he fears moral implications. In these lines, he fears the retribution he might face if he is caught:
Bloody instructions, which, being taught, return
To plague the inventor: this even-handed justice
Commends the ingredients of our poison’d chalice,
To our own lips.
Then Macbeth indicates that Duncan is trusting. Macbeth and Duncan are cousins (kinsman). Macbeth is his host and loyal subject. Duncan has every reason to trust Macbeth. This indicates that Duncan is vulnerable and an assassination would be easy. But because of this moral conflict, Macbeth decides not to go through with it:
He’s here in double trust;
First, as I am his kinsman and his subject,
Strong both against the deed; then as his host,
Who should against his murderer shut the door,
Not bear the knife myself.
But the main purpose of this monologue is to illustrate that Duncan is trusting and virtuous. Macbeth fears eternal damnation, which is more evidence that he is morally conflicted. Killing a virtuous person like Duncan would alert the heavenly authorities:
So clear in his great office, that his virtues
Will plead like angels, trumpet-tongued, against,
The deep damnation of his taking-off;
Here, “taking-off,” means to be killed and to take one’s possessions (the crown). If Macbeth were to kill Duncan, the angels (cherubim) “Shall blow the horrid deed in every eye,” meaning Macbeth would face moral and eternal consequences.
Macbeth concludes that the only incentive (spur) prompting him to consider killing Duncan is ambition. His ambition “overleaps itself,” and “falls on the other,” indicating his ambition is wild. Directed outwardly, it will manifest in violence.
After this speech, Lady Macbeth’s influence and his own ambition override this moralizing.