In Shakespeare's Macbeth, I believe that the one person he cannot kill is Banquo. Duncan is Macbeth's friend, his cousin and his King, but Macbeth is able to kill him only because of his wife's harassment, and his ambition to be king is so great.
Macbeth decides to kill Banquo without telling his wife. His murderous behavior is escalating. However, Banquo is not only his best friend, but he is a man of honor and integrity. I don't think that Macbeth can face his friend—I cannot believe he would be able to look his friend in the face and kill him: Macbeth was a brave warrior for Scotland, but in murdering others, he becomes cowardly. Macbeth also wants Fleance killed. Macbeth has only killed Duncan's guards since Duncan's death and admits that killing still bothers him because he is so "young" to it (lacking experience).
It would have been so much easier as the King to hire desperate men, convince them that Banquo was their enemy, and kid himself to believe his hands would not be bloodied by the actions of the murders. (Of course, as soon as Banquo is dead, his ghost appears to Macbeth to haunt him, freaking him out.)
The thing that Macbeth cannot do to Banquo is kill him.