There are several reasons Macbeth hesitates to kill Duncan, and he clarifies these for the audience in a soliloquy in act I, scene vii.

Macbeth begins by saying that he knows there will be consequences for his actions. He suggests that he would not be so anxious about murdering the king if the murder would "be the be-all and the end-all" (line 5). However, even putting aside the inevitable damning of his soul in the afterlife, Macbeth knows that he will deal with repercussions in his mortal life, as well:

But in these cases

We still have judgment here, that we but teach

Bloody instructions, which, being taught, return

To plague th' inventor: this even-handed justice

Commends the ingredients of our poisoned chalice

To our own lips (7–12).

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 (13–20) . . .

Macbeth is aware that murdering king Duncan would be one of the greatest sins and, he is talking about it in Act I, Scene 7:

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.

Macbeth states that going after his unchecked ambition which entails killing Duncan is wrong for three major reasons. Firstly, Macbeth is Duncan's relative, so it is abnormal and sinful to murder him since they are related. Secondly, Macbeth should be Duncan's most loyal subject, so he should fight for him, not kill him. Thirdly, when Duncan arrives at Macbeth's home, Macbeth will be his host, so he should protect Duncan because he will be Macbeth's guest. Macbeth should guarantee that his guest is safe. Additionally, Macbeth proclaims that the king is benevolent, and he has been recently promoted by the king.

All of the reasons listed above are strong enough, and Macbeth should not go against them. Yet, he states that his ambition is the chief reason why he wants to take the throne and eliminate Duncan.