If it were done when ’tis done, then ’twere well
It were done quickly. If th’assassination
Could trammel up the consequence, and catch
With his surcease success: that but this blow
Might be the be-all and the end-all, here,
But here upon this bank and shoal of time,
We’d jump the life to come. But in these cases
We still have judgement here, that we but teach
Bloody instructions which, being taught, return
To plague th’inventor. This even-handed justice
Commends th’ingredience of our poisoned chalice
To our own lips. 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. Besides, this Duncan
Hath borne his faculties so meek, hath been
So clear in his great office, that his virtues
Will plead like angels, trumpet-tongued against
The deep damnation of his taking-off,
And pity, like a naked new-born babe,
Striding the blast, or heaven’s cherubin, horsed
Upon the sightless couriers of the air,
Shall blow the horrid deed in every eye
That tears shall drown the wind. I have no spur
To prick the sides of my intent, but only
Vaulting ambition which o’erleaps itself
And falls on th’other.

In this soliloquy, which is found in Act 1, scene 7, lines 1–28, Macbeth debates whether he should kill Duncan. When he lists Duncan’s noble qualities (he “[h]ath borne his faculties so meek”) and the loyalty that he feels toward his king (“I am his kinsman and his subject”), we are reminded of just how grave an outrage it is for the couple to slaughter their ruler while he is a guest in their house. At the same time, Macbeth’s fear that “[w]e still have judgement here, that we but teach / Bloody instructions which, being taught, return / To plague th’inventor,” foreshadows the way that his deeds will eventually come back to haunt him. The imagery in this speech is dark—we hear of “bloody instructions,” “deep damnation,” and a “poisoned chalice”—and suggests that Macbeth is aware of how the murder would open the door to a dark and sinful world. At the same time, he admits that his only reason for committing murder, “ambition,” suddenly seems an insufficient justification for the act. The destruction that comes from unchecked ambition will continue to be explored as one of the play’s themes. As the soliloquy ends, Macbeth seems to resolve not to kill Duncan, but this resolve will only last until his wife returns and once again convinces him, by the strength of her will, to go ahead with their plot.