In act 1, scene 7, Macbeth delivers a very powerful and important soliloquy, in which he expresses his thoughts and emotions regarding the potential murder of King Duncan. He starts it off by saying that if he were to murder the King, he should do it swiftly and as quickly as possible.

If it were done when 'tis done, then 'twere well

It were done quickly. If the 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.

This actually reveals Macbeth's uncertainty about committing the horrible crime, and this is very relevant, as it showcases his moral compass. Some would even argue that it represents the corruption of his morality and the possible destruction of his conscience. Macbeth entertains the possibility that he might cope better with his emotional unrest and face the inevitable consequences of his actions.

Macbeth recognizes the fact that murdering King Duncan will bring even more injustices in the kingdom, as many might follow his example, and that the deed will basically damn his soul. He knows that all of this will eventually destroy him psychologically. He acknowledges that the king is a kind man and a graceful and virtuous ruler and that the kingdom will be terribly saddened by his death. Moreover, the king is also his friend, and the two are actually fond of each other and trust one another.

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.

In the end, Macbeth decides that the only reason and motive he has to murder King Duncan is his "vaulting ambition" and decides not to do it.

And pity, like a naked newborn 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 th' other.

However, Lady Macbeth manages to convince him to go through with the nefarious plan and murder the king by emotionally manipulating him and making him forget about his doubts and his humanity.

I would like to add a few things to the above answers.  

This first line of the soliloquy begins with the key word "if" and kicks off Macbeth's thought process about his actions and their future repercussions.

Yes, Macbeth is arguing with himself over whether or not he should kill Duncan at all, and if so, when. The portion of the soliloquy which immediately follows your given quotation justifies this concept:

...: if the assassination / could trammel up the consequence and catch / with its surcease, success; that but this blow / might be the be-all and end-all here (I, vii, 2-5)

However, there is another aspect to the quotation which gives us a look into Macbeth's logic. First Macbeth notes:

But here upon this bank and shoal of time, / we'd jump the life to come (I, vii, 6-7)

Here, Macbeth shows that he understands the trade-off he is making for a life of wealth and power.  By "jumping the life to come," Macbeth would be trading his life on earth for his afterlife. While his earthly life may be everything he wants, his afterlife will no longer be the heaven he has imagined.

To him, this is no small choice. In addition, the next several lines of the soliloquy discuss what could go wrong on this earth. With the words

Bloody instructions, which, being taught, / return to plague the inventor:... (I, vii, 10-11)

Macbeth realizes that if he kills a king for wealth and power, there is nothing to stop another from doing the same to him, thus hastening his journey to an afterlife now spoiled with his sin.

All in all, Macbeth is not just discussing the timing of his deed, but considering the ramifications both on earth and beyond before he makes his deadly choice.

In this very dramatic soliloquy (scene 7, Act 1), Macbeth is clearly troubled by what he and his wife have planned to do: the murder of king Duncan. Macbeth's conscience is gnawing at him and he is expressing doubt about going through with the deed. In scene 5, Lady Macbeth had already mentioned that:

"He that's coming
Must be provided for: and you shall put
This night's great business into my dispatch;
Which shall to all our nights and days to come
Give solely sovereign sway and masterdom."

This of course, is a direct reference to Duncan's arrival and his murder as well as their success.

Macbeth is saying here that if the foul act of Duncan's assassination has been committed and is over and done with, it would be good if it were done as soon as possible and quickly. This suggests that Macbeth sees the task as so unpleasant that it would be best to get it over with. There should be no delay.

Furthermore, Macbeth says that if Duncan's murder could have no negative consequences, it would be good and if his death could be the end of the whole sorry saga and ensure his successful claim of the throne, it would be a definite boon. However, he realises that there will be consequences:

"But in these cases
We still have judgment here;"

In the rest of the soliloquy, Macbeth expresses a variety of reasons why Duncan does not deserve such an ignominious death: he is both related to Duncan and is his subject, he is Duncan's host, Duncan is a virtuous king, he is much loved by his subjects and there would be a terrible outcry. Finally, the only purpose for him to murder his king would be his "vaulting" ambition.

Immediately after the soliloquy, Lady Macbeth approaches him and he, convinced now that he does not want to continue with the plot, declares:

"We will proceed no further in this business"

Lady Macbeth is, of course, shocked by her husband's sudden turnaround and proceeds to persuade him, but that is a topic for another essay.

Macbeth is arguing with himself again about whether to murder King Duncan. If it could be done without causing problems later, then it would be good to do it soon. If Duncan's murder would have no negative consequences and be successfully completed with his death (surcease), then Macbeth would risk eternal damnation. He knows, however, that terrible deeds (bloody instructions) often backfire.

"If it were done when 'tis done" means if the deed were at an end, completely finished, at the moment it is done; Macbeth thinks that if he could avoid (”trammel up” means catch in a net) the consequences of murder with the death (”surcease”) of Duncan, he would take a chance on the life to come (on earth, and in heaven or hell).

text of macbeth