Macbeth worries about getting caught, feels Duncan has not done anything to deserve being killed, and believes a host should not kill a guest.
The first problem Macbeth has is that killing a king is very serious. He tries to talk himself out of it in his soliloquy. He begins by saying that, if he does kill Duncan, then he needs to do it quickly. Killing the king is dangerous. Macbeth is worried about the effects it will have on the kingdom. Will one murder create a domino effect of murders?
There are additional problems because the king is Macbeth's kinsman and his guest.
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 I, Scene 7).
Basically, Macbeth would be violating every rule of gracious hosting by killing Duncan while he is staying at his home. You are supposed to protect your guests, not kill them. This is even worse when your guest is also your kinsman.
Macbeth then goes on to praise Duncan, saying he is great and doesn’t 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 I, Scene 7).
Duncan has been a noble and virtuous leader. People will sing his praises when he dies. He doesn’t deserve death. It sounds like Macbeth is no ethical match for him. Therefore, Macbeth worries about killing and serving as the replacement for such an excellent leader. Macbeth's only qualification for being king is that he wants to be king badly enough to kill the current king.