Macbeth tries to talk himself into killing Duncan, but he does not want to because Duncan has promoted him, because they are kinsman, and because Duncan is his guest.
Macbeth is concerned about killing Duncan because he is his kinsman and his subject. He should be loyal to Duncan, not murder him.
He's here in double trust:
First, as I am his kinsman and his subject,
Strong both against the deed (Act 1, scene 7)
Macbeth notes that he is also Duncan’s host, and it is his responsibility to “shut the door” against his murderer, not murder him himself.
Macbeth also notes that Duncan is a good king, and does not deserve death.
Besides, this Duncan
Hath borne his faculties so meek, hath been
Will plead like angels trumpet-tongued against
The deep damnation of his taking-off (Act 1, scene 7)
Duncan has done nothing to cause Macbeth to kill him. He has been a loyal subject to the king, and there is no reason for the king to suspect otherwise. It is only Macbeth’s ambition that causes him to murder Duncan. His ambition, and his wife’s prodding.
Macbeth mentions that Duncan is at his castle in "double trust," which means that Duncan has two reasons to trust Macbeth, and that Macbeth will do him no harm and will protect him.
Firstly, Duncan is his "kinsman," which means that they are related to one another, not only because they are citizens of the same country, but also in blood. Duncan and Macbeth are cousins, all the more reason for Macbeth not to harm him – such a deed would be unnatural in the extreme.
Secondly, Macbeth is Duncan's "subject," and as his subject, he has to honor his king and show allegiance and loyalty. A loyal subject should not even consider the thought of bringing harm to his king, let alone plot his assassination.
These two reasons on their own speak "strong ... against the deed." They are more than ample conviction for Macbeth not to kill Duncan.
Furthermore, Duncan is also Macbeth's guest, and it is imperative for a host to ensure the safety of his guests. It would go against the grain if the host should consider committing any dark, obnoxious deed against his guest/s.
Also, Duncan has been a kind and gentle king. He has taken care of his subjects and has not been a tyrannical and despotic leader. He has been gentle and "clear" in his "great office" which implies that his actions were never questionable – further evidence that killing him would be a purposeless exercise.
Because Duncan is such a good king, there would be a clamor to bring the murderer/s to justice. There would be a cry in heaven against the assassination. Macbeth would be damned forever – more than enough reason to not even consider Duncan's assassination. Macbeth also realizes that not only would he be damned, but also that those loyal to Duncan (which more than likely would be practically the whole of Scotland) would seek revenge against the callous murderer.
These are the reasons Macbeth provides against killing Duncan. The only motive for him to continue with this dastardly deed would be his unbridled ambition and greed.