As Macbeth thinks about what the witches have promised, what "horrid image" frightens him? Why?

That would be killing Duncan. Macbeth really does not want to kill King Duncan. The prospect of killing the king scares Macbeth,

If good, why do I yield to that suggestion
Whose horrid image doth unfix my hair
And make my seated heart knock at my ribs,
Against the use of nature?