What moral reasons does Macbeth give for not assassinating Duncan in Macbeth?

Macbeth says that he should not kill Duncan because he is his guest and kinsman.

Macbeth has wanted to be king ever since he heard the witches’ prophecies.  He also wrote a letter to his wife telling her about the witches’ predictions that he would become king, so she is on board and ready to help him dispatch Duncan if he can’t become king by legal means.

Lady Macbeth fears that her husband is too full of “the milk of human kindness” to do what needs to be done and kill Duncan.  She may be right.  Almost as soon as Macbeth returns home he has second thoughts.  Macbeth goes down the list of reasons why he should not kill the king.  First he worries about the consequences and then he comes up with reasons not to do it.

Macbeth notes that Duncan trusts him, because they are related and because he is supposedly one of the king’s loyal subjects.  He should therefore be a good host, protecting the king rather than killing him.

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. (Act 1, Scene 7)

Macbeth also notes that Duncan is a good king, and does not deserve to die.

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 … (Act 1, Scene 7)

If Duncan were a tyrant or had mistreated Macbeth, then it might be easier to kill him.  Instead, Macbeth feels that it would be immoral to kill a virtuous king for no reason just because he wanted to be king himself.

In this speech, Macbeth makes a great change from his fiery aside when he found out Malcolm was Duncan’s heir and called Malcolm “a step/On which I must fall down, or else o'erleap,/For in my way it lies” (Act 1, Scene 4).  At that point he seemed ready to do whatever it took to become king.  Once Duncan was actually in his house, however, he found betraying him more difficult.