Macbeth is disturbed by the idea of killing Duncan in order to be king.
When the three witches deliver their prophecies and one comes true, Macbeth is excited. He is pleased with the idea that he is Thane of Cawdor, and has a chance to be king. At the same time, he shrinks from the idea of killing Duncan. It makes his heart beat fast to think of it.
Are less than horrible imaginings:
My thought, whose murder yet is but fantastical,(150)
Shakes so my single state of man that function
Is smother'd in surmise, and nothing is
But what is not. (Act 1, Scene 3)
Even considering the proposition terrifies him. He does not see himself as a murderer, and it certainly never seemed to occur to him to murder Duncan. He says later that he loves Duncan, and considers him a kinsman.
Macbeth’s indecision is a running theme. His wife convinces him to kill Duncan, but even then he wavers. After the bloody deed is done, he continues to have regrets that seem to lead him to slowly lose his mind (while his wife more quickly loses hers). It shows us Macbeth is not all bad.