It is not accurate to say that Macbeth gave no thought to what he would do about Malcolm and Donalbain when he killed their father. Duncan announces that he is making Malcolm the Prince of Cumberland, which means the young man is heir to the throne. In Act 1, Scene 4, Macbeth says to himself:
(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.
Shakespeare inserts these lines because he just does not want to deal with the question of what Macbeth intends to do about Malcolm and Donalbain, both of whom stand ahead of him in the line of succession, and the elder of whom has been officially and publicly acknowledged as next in line by his father the King. The most important line in the passage quoted above is: "Let not light see my black and deep desires." Macbeth is saying, in effect, that his plans regarding Malcolm and Donalbain are made but that they will remain completely hidden until he has disposed of their father. The passage also suggests that Macbeth doesn't like to think about killing a couple of young boys. This is Shakespeare's way of dealing with an extremely complex matter by "shelving it," so to speak, "by putting it in the closet" with the intention of dealing with it later.
Shakespeare already has too much to deal with in dramatizing the murder of Duncan. Probably when he was writing the play he told himself he would worry about what to do with the two sons after he had written everything up to Duncan's murder and Macduff's discovery of the body. In other words, Shakespeare himself didn't know what Macbeth intended to do about Malcolm and Donalbain, but he pretends in the lines quoted above that Macbeth and his wife have discussed the matter thoroughly--as well they should have done!--and that they have a plan.
Shakespeare seems to have written his plays under time pressure and to have relied on inspiration, luck, and what he himself called "the virtue of necessity" to help him out in dealing with the problems he himself had created earlier. Shakespeare knew he was a genius and that he could always come up with a solution to a plot problem if he found that he had painted himself into a corner, so to speak.
Then Shakespeare has Malcolm and Donalbain decide to flee for their lives. This is something Macbeth could not have foreseen, but it enables him to pin their father's murder on the boys and "o'erleap" both of them to become king. That was the best idea that Shakespeare could come up with. The only alternative would have been to have Macbeth and his wife kill all three of their guests--Duncan, Malcolm, and Donalbain. And Shakespeare did not want to do that for many reasons. For one thing, he wanted Malcolm to raise an army and come back to claim the throne. For another, it just seemed unworkable to stage three murders in one night when it was complicated enough to stage even the one. He dismisses any possibility of Macbeth going on to kill the boys in their beds by creating voices, knocking at the gate, and giving Macbeth a sort of nervous breakdown. And for another reason, Shakespeare wanted his audience to feel at least a little sympathy for Macbeth. The audience would lose all sympathy for him if he even talked about murdering a couple of innocent young boys in their sleep.
There are probably many other reasons why Shakespeare virtually ignored the question of what Macbeth planned to do with Malcolm and Donalbain after killing their father. Another important reason is that Shakespeare was more or less stuck with facts of history. He just couldn't kill Malcolm if Malcolm hadn't really been killed by Macbeth in actual historical fact. Besides that, how would Shakespeare end his play? He had to have somebody come to Scotland with an army and dethrone Macbeth.
The prophecy of the witches does not mention anything about Malcolm or Donalbain. The prophecy only mentions that he (Macbeth) will be king. That said, Banquo's prophecy states that his sons will be kings--no mention of Duncan's sons at all. Therefore, one could assume that Macbeth's concerns lie with Duncan and Banquo (and Fleance) alone. Now that is it mentioned though, I never did consider if Macbeth thought about Duncan's sons' (specifically Malcolm) heir to the throne. I do think that Macbeth only considers Duncan and nothing more. He failure to plan well leaves many holes in the murder of Duncan and the time following the finding of Duncan's body.
Macbeth is a play about how dreams become nightmares, something becomes nothing and nothing is "what is not." Reality and the phantasmorgic are constantly shifting, so that there is a complex succession of things imagined and things seen and things done. So, after Macbeth imagines the bloody dagger before him as he contemplates the murder of his cousin, King Duncan, he foresees the bloody sequence of his plan, and he senses the preternatural, as well as contemplating his upsetting of the Chain of Being:
...wicked dreams abuse
The curtained sleep; witchcraft celebrates
Pale Hecate's offerings; and withered murder,
Alarumed by his sentinel, the wolf, ....
Moves like a ghost....(2.1.76-81)
After having committed the murder, in Act II, Scene 1, Macbeth returns to his wife, ridden with fear and guilt. While he does ask Lady Macbeth, "Hark!/Who lies i'th'second chamber?" and she tells him that it is Donalbain, Macbeth's only reply is, "This is a sorry sight." Perhaps because in Act I the witches have foretold nothing about Duncan's sons, but instead have spoken to Banquo of his sons becoming kings, Macbeth thinks not of Donaldbain and Malcolm. Certainly, as literary critic William Hazlitt writes of Macbeth, "He is sure of nothing but the present moment." And, at this point in the tragedy, Macbeth's thoughts are absorbed with his regicide which has become vivid in his own imagination:
Methought, whose murder yet is but fantastical
Shakes so my single state of man that function
Is smothered in surmise, and nothing is
But what is not. (2.1.42-45)
Macbeth's primary intention is to kill Duncan. The act of murdering Duncan itself is very intense for Macbeth.It casts such a large shadow upon Macbeth that he could not conceive of engaging in any other act of cruelty. Consider what Macbeth says when he returns from the deed upon seeing Lady Macbeth. The text indicates that Macbeth ends up hearing voices, projections from his own conscience about him killing Duncan: "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." This demonstrates that Macbeth was wracked with guilt and regret about Duncan's murder. This also means that Macbeth would not have had the moral or ethical capacity at this point to kill more people. He could not have brought himself to killing Donalbain and Malcolm. To kill the king's sons would have required a capacity that Macbeth at this point in the drama does not possess. This level of guilt and sense of sadness Macbeth experiences is also seen in “Wake Duncan with thy knocking! I would thou/ couldst!” This is textual evidence that confirms Macbeth lacking the ability to put aside one murder, making him incapable of intending to kill more.
In a condition filled with guilt and regret, it is not feasible to see Macbeth being able to kill two more people. As the narrative develops, the ability to commit murder and to accept it as a part of consciousness is what causes his characterization to change. However, at the point in which Duncan is killed, there is sufficient textual evidence to suggest that Macbeth could not have brought himself to kill Donalbain and Malcolm. The sons themselves recognize that their father's murderer is someone close to the king, someone who deliberately means harm for them as well as their father: "This murderous shaft that’s shot/Hath not yet lighted, and our safest way/Is to avoid the aim.” Their realization at the need to escape is another example of why Macbeth could not envision killing the sons. Even if his intent might have extended to this point, the reality is that the window of opportunity closed with the father's murder. Macbeth never originally intended to widen his actions. Lady Macbeth suggested the target of Duncan, and this was the only focus that Macbeth had. From this, the text indicates that Macbeth could not have surmised the need to kill Malcolm and Donalbain. Macbeth's "heat- oppressed brain" conceives of only killing Duncan, an act that precludes any other thoughts Macbeth might have had of murdering more people.
I realize that Macbeth was so overwhelmed with emotions during and after murdering King Duncan that it would not have been possible for him to commit two more murders. However, what I am trying to ascertain is whether or not he intended to kill Malcolm and Donalbain that same night, whether he actually did so or not. If he didn't have any thoughts about killing Malcolm and Donalbain, then how did he expect to succeed Duncan as king? What was the game plan, if any? Did he discuss it with his wife? Duncan had previously specified that Malcolm was the heir apparent to the throne. Macbeth had no way of knowing that Malcolm and Donalbain would decide to flee for their lives, giving him an opportunity to blame their father's murder on them.