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.