Malcolm seems like an ineffectual character in many ways. He does not become effectual until almost the end of the play when he returns to Scotland at the head of the English army.
Early in Macbeth, Shakespeare leaves an important question unanswered. After Duncan’s body was discovered, why didn’t Malcolm, his eldest son and heir apparent, stay and claim the crown instead of fleeing to England? Duncan had named Malcolm the Prince of Cumberland and publicly proclaimed him his heir.
Sons, kinsmen, thanes,
And you whose places are the nearest, know
We will establish our estate upon
Our eldest, Malcolm, whom we name hereafter
The Prince of Cumberland
It was very convenient for Macbeth that both Malcolm and his brother Donalbain, next in line of succession, should have decided to flee, because the assassin could blame Duncan’s murder on them and claim the crown himself—but he couldn’t have foreseen this development. Another unanswered question is: Why didn’t Macbeth murder Duncan’s two sons in their sleep on the same night he murdered their father?
In Act 4, Scene 3, Shakespeare seems to be tying up loose ends. In the very strange conversation between Malcolm and Macduff, the younger man accuses himself of all kinds of faults which he feels make him unworthy to be King of Scotland. At one point he says: “…better Macbeth / Than such an one [as himself] to reign.” All of this confession would seem intended to explain why Malcolm did not stay and claim the crown on the morning of his father’s murder. But then he disclaims all his self-accusations and tells Macduff that he was only testing his loyalty because he suspected him of being an agent of Macbeth. This seems little short of preposterous. Even if he suspected Macduff of being a spy for Macbeth, it was very bad form to say so to a thane who had come there to help him.
The only explanation for the inclusion of this long and unproductive scene is that Shakespeare wanted to explain why Malcolm didn’t claim the crown in the first place. Evidently he felt unworthy to be king. But what is he doing in England trying to raise an army of ten thousand men to invade Scotland? The other apparent purpose of this scene is to bolster Malcolm as a character in the minds of Shakespeare’s audience. He had very little to do or say while he was at Dunsinane, but he has to become Macbeth’s chief antagonist and leader of the invading army. The scene gives the audience an opportunity to familiarize themselves with this character. It also gives the audience further opportunity to become better acquainted with Macduff, who will be the one to deliver Macbeth’s death blow.
Otherwise, the scene seems like a waste of words. Malcolm says he is unworthy to have claimed the crown in place of Macbeth, and then he disclaims every bad thing he said about himself and makes himself appear almost as pure as the saintly English King Edward.
Scene 3 of Act 4 is never included in film adaptations of Macbeth. It is quite sufficient for the viewer to understand that Malcolm has fled to England, raised an army, and come back with Macduff to claim the Scottish throne. The question of why Malcolm fled from Scotland in the first place remains unanswered. The viewer might accept his explanation that he didn't feel worthy to become king because of all his vices if he didn't disclaim all those vices practically in the same breath.