Macbeth has no reason to assume he could become king simply by assassinating Duncan. In fact, when the Three Witches suggest it, he says such a “prospect” is out of the question.
By Sinel’s death I know I am Thane of Glamis. But how of Cawdor? The Thane of Cawdor lives A prosperous gentleman, and to be king Stands not within the prospect of belief, No more than to be Cawdor (Act I, Scene 3).
The main reason Macbeth thinks his prospect of becoming king is infinitely remote is that Duncan has two sons who are both quite properly ahead of him in the line of succession. When Duncan proclaims Malcolm the Prince of Cumberland, it is a guarantee that the elder son will be king after Duncan. If anything should happen to Malcolm, there is still Donalbain to become heir apparent. This raises the unavoidable question of why Macbeth does not plan to kill Duncan, Malcolm, and Donalbain all on the same night. To put it another way, what can Macbeth hope to achieve if he kills Duncan and leaves the sons alive?
Shakespeare could not avoid the problem altogether. His audience would naturally be wondering what Macbeth intended to do about the sons. Act I, Scene 4 has the only hint. Macbeth says to himself,
The Prince of Cumberland. That is a step On which I must fall down or else o’erleap, For in my way it lies. Stars, hide your fires; Let not light see my black and deep desires. The eye wink at the hand, yet let that be Which the eye fears, when it is done, to see.
What these cryptic words suggest is that Shakespeare wanted to give his audience some assurance. Macbeth realizes he has to do something about Duncan's sons and has plans to dispose of them, but does not want to think about it at this point. Macbeth does not like murdering a couple of young boys in their beds, but he believes that is what he is going to have to do after taking care of their father. It is hard enough for Macbeth to commit that one awful murder, as we see, but to have to think about committing three murders in one night is more than his mind or conscience can handle.
As it turns out, Macbeth had to retreat to his bedchamber after killing the King. He could not have killed the sons, if that is what he intended to do, because he had something like a nervous breakdown after committing the first atrocity. To his credit, he is not a cold-blooded killer. He could not just go from chamber to chamber killing off an entire family. Furthermore, he imagined he heard a voice shouting loud enough to wake the entire castle.
Methought I heard a voice cry “Sleep no more! Macbeth does murder sleep”—the innocent sleep, Sleep that knits up the raveled sleave of care, The death of each day’s life, sore labor’s bath, Balm of hurt minds, great nature’s second course, Chief nourisher in life’s feast.
Still it cried “Sleep no more!” to all the house. “Glamis hath murdered sleep, and therefore Cawdor Shall sleep no more. Macbeth shall sleep no more” (Act II, Scene 2).
A few lines later, there begins the terrible knocking at the gate, which Thomas De Quincey discusses in his famous essay. There are at least three reasons why Macbeth cannot murder Malcolm and Donalbain, assuming that was what he intended to do:
- He loses his nerve.
- He thinks he hears a loud voice crying “Sleep no more!” to all the house—which is the same as crying “Wake up, everybody!”
- There is a knocking at the gate which goes on and on until the drunken Porter finally opens it in the next scene; and Macbeth—far from being able to murder two young men who may already be awake, far from being able to pretend to have been asleep in bed while Duncan was being murdered—is forced to put in a personal appearance and conduct Macduff and Lennox to the scene of the crime.
Macbeth has no way of foreseeing that Malcolm and Donalbain would decide to flee for their lives, enabling him to pin their father’s murder on them. Their decision is plausible because they suspect there may be a widespread conspiracy against them involving many of the assembled thanes. They are certainly right to think their lives are in danger. They don’t know how close they may have come to being killed in their sleep.