If it were done when ’tis done, then ’twere well
It were done quickly: if the assassination- IAMB weak foot
Could trammel up the consequence, and catch – IAMB  
With his surcease success; that but this blow – IAMB
Might be the be-all and the end-all here, 
But here, upon this bank and shoal of time, 
We’ld jump the life to come. But in these cases 
We still have judgment here; that we but teach 
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. 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 cherubim, 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 the other.

Macbeth, is in this soliloquy, speaking his thoughts aloud about how it is best to deal with Duncan. He starts by informing us that he is not going to enjoy killing Duncan but that if he must he would hope it was done quickly and cleanly. He intones….

“If it were done when ’tis done, then ’twere well It were done quickly:…”

Duncan is a nobleman of great valor and honor. In his time he has been a just and good king. As Macbeth keeps talking he realizes that to kill such a great man would not be a great crime not only literally but also morally. Beginning to freak out even more at the idea of killing the king he speaks aloud his thoughts that Duncan is “….here in double trust First, as I am his Kinsmen……; then, as his host, Who should against his murderer shut the door”. Shakespeare use’s an amazing metaphor in this quote. The idea of shutting “the door” on his guest’s murder’s.