In Macbeth, why does Macbeth think that he should not kill King Duncan?
There are many reasons that Macbeth reviews in his mind and in discussion with Lady Macbeth as to why he should give up the plan to kill Duncan. As has been mentioned, these include the personal excellence of Duncan, Duncan's trust in him, and Duncan's position at the moment as a guest in Macbeth's castle. However, the most important reason -- even more important than the prospect of eternal damnation for such a murder -- is given by Macbeth in Act I, Scene 7:
But in these cases
We still have judgement 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.
In other words, if Macbeth murders Duncan, he establishes a precedent that will come back to haunt him: his deed will "plague the inventor." How can he ask anyone to be loyal to him if he has gained the throne by disloyalty and so "taught" the "bloody instructions" that a superior is to be stabbed in the back whenever it is convenient? The details of how he would fail his trust -- turning on a superior, harming a guest -- are less important than the fatal example he would give by betraying trust in any way at all.
Macbeth has high regard for King Duncan. He knows that Duncan has been an excellent king, wise and fair, and that the people love him. Macbeth says that "tears shall drown the wind" when Duncan dies.
Macbeth knows that Duncan trusts him and values his service. Shortly before Duncan is murdered, he had made Macbeth Thane of Cawdor to reward him for his loyalty and bravery in fighting against the forces of the King of Norway that had attacked Scotland. Finally, Duncan is a guest in Macbeth's castle. It is Macbeth's responsibility as Duncan's host to protect him, not kill him.