Why doesn't Macbeth kill Malcolm and Donalbain?
Macbeth has a hard enough time killing the king, Duncan, let alone his two sons, Malcolm and Donalbain. He hallucinates a dagger that turns bloody before his eyes prior to killing the king, and he chalks the hallucination up to his "heat-oppressed brain": he is obviously so stressed and overwhelmed that I'm not actually sure it's even occurred to him to think about Malcolm and Donalbain. He and Lady Macbeth have not made any plans for the king's sons, only for the king's chamberlains, who they frame for the murder. After Duncan's body is discovered the next morning, however, Malcolm and Donalbain talk, agreeing that
To show an unfelt sorrow is an office
Which the false man does easy. (2.4.161–162)
In other words, they think it is easy to pretend to grieve and feel sad when one does not actually feel it; they are aware of the fact that their father's killer could be among those who profess to be their loyal friends. Therefore, Malcolm decides to flee to England and Donalbain to Ireland, hoping that splitting up will "keep [them] both the safer" (2.4.164). Once the king's sons flee the country, Macbeth can no longer reach them.
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It's enough for Macbeth to kill Duncan (and Duncan's servants) as one night's work.
We remember that Macbeth still has a conscience at this point. He comes back to his chambers and his wife is stunned and horrified at the murder he has committed, almost ready to go over the edge as he contemplates how much blood came from Duncan's body. Continuing his murderous rampage is definitely not part of his evening plans—and it's not likely he is capable of it at this point. Further, Malcolm and Donalbain are the most likely murder suspects, as they are next in line to the throne, so keeping them alive deflects suspicion from Macbeth.
Macbeth and Lady Macbeth seem not to have thought through their plan too carefully, as they are so anxious to kill Duncan while they have the chance. However, circumstances work in their favor. When Malcolm and Donalbain realize what has happened, they also realize they are likely to be the next targets. Therefore, they flee Scotland, which makes them look very guilty. It also clears the path for Macbeth, Duncan's cousin, to take the throne.
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Macbeth and his wife plot to murder King Duncan while they have him sleeping under their roof for the first and probably the only time. They talk as if Duncan’s death will somehow automatically make Macbeth king. What about Malcolm and Donalbain, though? Malcolm is next in line of succession. Shakespeare specifically points this out: Duncan announces he is appointing Malcolm the Prince of Cumberland, which makes him heir apparent to the Scottish throne.
Malcolm and Donalbain are also sleeping in Macbeth’s castle for that one night only. He has no way of foreseeing that they will decide to flee for their lives after their father’s mutilated body is found. It seems that Macbeth must be planning to murder both of them on the same night he kills their father—but he and his wife say nothing about the two sons. The only indication in the text that Macbeth intends to dispose of Malcolm and Donalbain is in his reaction to the appointment of Malcolm as Prince of Cumberland.
[aside] 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.
Macbeth seems to be telling himself that he doesn’t even want to think about what he is going to do to Malcolm, but that he nonetheless has made his plans. If he kills Malcolm, he would also have to kill Donalbain; otherwise, the younger son would become next in line for succession. The crucial words in the above lines are “yet let that be.” Not only does Macbeth have plans for the boys, but he and his wife must have discussed them thoroughly. Shakespeare may have “finessed” the whole problem for at least two reasons. One was that it would complicate the drama. The other was that Macbeth would lose audience sympathy if he was thought to be planning to kill two boys in their sleep. Shakespeare wanted to preserve some modicum of sympathy for his titular character.
Shakespeare invents several reasons why Macbeth could not murder the sons along with their father, assuming that was what Macbeth planned to do. He returns to his chamber holding two bloody daggers. The audience might think he had killed the whole family, but he tells his wife:
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.
This beautiful tribute to sleep seems intended to explain that Macbeth was afraid of getting caught with the two daggers in his hands and decided to abort the plan to kill the sleeping boys. He thought the imaginary voice was going to wake up all the household and he would be found in flagrante delicto. If the voice was going to wake people up, it could easily wake Malcolm and Donalbain.
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.”
Macbeth was already unnerved before he thought he heard the voice crying “Sleep no more!” He tells his wife that when he was in Duncan’s chamber he might have been seen by both of the guards.
One cried “God bless us” and “Amen” the other, As they had seen me with these hangman’s hands, List’ning their fear.
Then, just as Lady Macbeth exits to return the daggers to Duncan’s chamber and smear the faces of the sleeping grooms with the King’s blood, there comes the first “Knock within.” The prolonged knocking, which became the subject of a famous essay by Thomas De Quincey, has many functions, the first being to make it obviously impossible for Macbeth to murder Malcolm and Donalbain—assuming that was his intention. The knocking continues through the rest of Act II, Scene 2, and is not explained until the the drunken Porter enters in Scene 3. De Quincey apparently thought of it as only a stage effect, but I would argue Shakespeare intended it to prevent Macbeth from considering killing Duncan’s two sons and also to force Macbeth to go down in his nightshirt to find out why nobody was opening the gate. This meant that—much against his will and contrary to his original plan—Macbeth had to be present when Duncan’s body was discovered by Macduff, who immediately raised such a clamor that he woke up everyone in the castle. It isn’t until Malcolm and Donalbain appear onstage that the audience knows whether they were alive.
The two boys were not killed because Shakespeare did not want them killed. He especially wanted Malcolm to flee to England and raise an army to overthrow King Macbeth. Shakespeare seems to have wanted to leave the impression that Macbeth actually did intend to kill the boys but was prevented from doing so by his own imagination and by the knocking at the gate.
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