Macbeth reveals his doubts about murdering Duncan in a soliloquy in Act I, Scene 7. He acknowledges that Duncan is a good king, and indeed a good man, whose murder may bring consequences in the afterlife. He also acknowledges that Duncan is at his castle "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.
Ultimately, Macbeth acknowledges that he has no reason to kill the king, except for his "vaulting ambition" which, he believes, can only be realized by murder. In fact, he seems to have decided not to go through with the murder when his wife arrives on the scene and goads him into overcoming his fears, questioning his manhood and his courage. After his wife encourages him to "screw your courage to the sticking-place," Macbeth says, "I am settled, and bend up/Each corporal agent to this terrible feat." His internal conflict resolved (at least in part,) he prepares to go through with the terrible deed.