Structurally, Act3.4 is a key point in Macbeth; furthermore, it is the point in which Macbeth's character reaches a turning point. Up to this time, with all his hesitation and wild fancies and gloomy suspicions, he has had strength of mind and self-control enough to push forward to his objects and to hide from public view the bloody means by which he has obtained them. In this scene, however, we see a fatal collapse of his powers.
Confronted by the spectre of his murdered victim he loses all self-control, and before the assembled nobility breaks out into speeches which must inevitably betray his guilt. It is interesting to compare his behaviour immediately after the discovery of the murder of Duncan with his actions in the presence of Banquo's ghost. In the former case he retained all his presence of mind; his speeches, though perhaps somewhat exaggerated, conveyed the impression of wild grief for the king's death, and his act of putting the bewildered grooms to instant death was, perhaps, the most practical thing that he could have done at such a time. In the banquet scene, after one feeble effort to play his part, he loses consciousness of the witnesses and speaks to the ghost as if they were alone together.
Equally noticeable is the fact that in this scene he passes altogether beyond his wife's control. She had been able to brace him up to the murder of Duncan and to control and direct him in the outburst of excitement which followed. In this scene, however, she is utterly unable to restrain him, and is forced to listen helplessly to the ravings that betray his guilty secret.
In the dialogue between Macbeth and his wife which follows the retirement of the guests, we see evident signs of moral degeneration as well as of the collapse of his mental powers. His expressed determination to seek out the witches and to wade through a sea of blood to obtain his objects...