Macbeth hesitates to kill Duncan because he has a change of mind. He begins thinking about the fact that Duncan has recently honored him by promoting him to the position of Thane of Cawdor. Macbeth explains to Lady Macbeth that he has decided not to kill King Duncan. In fact, Macbeth is quite adamant about his decision to not kill King Duncan:
We will proceed no further in this business.
He has recently honored me, and I now have the
Golden opinions from all sorts of people,
Which I want to enjoy for a bit longer, and
Not cast them aside so soon.
Macbeth seems content in being honored as Thane of Cawdor at this point. He is satisfied by having the golden opinions of all sorts of people. He admits that he does not desire to cast aside his his enjoyment so soon.
Of course, Macbeth's decision to not kill King Duncan is overruled by Lady Macbeth. She insults Macbeth's manhood. She calls him a coward. She manipulates Macbeth and insists that he follow through with the murder.
Macbeth is greatly influeneced by Lady Macbeth. He gives in to her manipulation. Macbeth becomes convinced to murder King Duncan. His actions are controlled by Lady Macbeth. She has more influence on him than he has upon himself.
Macbeth considers all the virtues Duncan has and the favors that the king has bestowed upon him, as well as the fact that he is related to the king and that Duncan is a guest in his castle. He also thinks about the repercussions his ghastly deed would have if he should follow through, such as the general outcry that will arise from such a malevolent act. He will be committing regicide and will be responsible for the death of a much-loved cousin and liege. There will be an outcry for justice and revenge.
Furthermore, a host is also responsible for taking care of the safety of his guests, not bringing them to harm! All these considerations are questions of morality and are disturbing to him. As such, they dwell on his mind and torment him. In his soliloquy in Act l, Scene 5, he provides all the reasons for not committing the act:
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.
Not only does he hesitate, but he goes so far as to tell Lady Macbeth that "we will proceed no further in this business," meaning he has decided not to go through with their pernicious plot. She, however, persuades him not to be a coward and push through and, in the end, Macbeth is persuaded by her emotive language.