Macbeth’s arguments for not killing Duncan include the fact that he is his cousin and subject, Duncan is his guest, and he should protect him.
After hearing the witches' prophecies, Macbeth decides he deserves to be king. He is angry when Malcolm is chosen to follow Duncan. He decides to do something about it.
Macbeth seems to be talking himself out of killing Duncan. Even though Lady Macbeth has a plan and is walking him through it, he has doubts. Maybe he really is “too full o’ the milk of human kindness” (1:5).
Macbeth expresses his doubts in a soliloquy.
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,(15)
Not bear the knife myself. (1: 7)
By “double trust” he means that he is Duncan’s cousin as well as his subject. He is also Duncan’s host. He notes that as host, he needs to protect Duncan. He should be preventing harm from coming to him, instead of bringing it to him.
Despite these concerns, it seems as though the bloody dagger will out. Macbeth, with his wife’s encouragement, decides that he will in fact kill Duncan because he deserves to become king.