Under the bitter irony of this speech we can see clearly enough that Macbeth has been exposed by his series of suspicious acts; he has 'done all things well;' and in particular by peculiar resemblances between this last incident of Banquo and Fleance and the previous incident of Duncan and his son. It appears then that Macbeth's last successful crime proves the means by which retribution overtakes all his other crimes; the latter half of the play is needed to develop the steps of the retribution, but, in substance, Macbeth's fall is latent in the final step of his rise. Thus the first half of the play, that which traces the rise of Macbeth, is a complete Nemesis Action—a career of sins in which the last sin secures the punishment of all.
The fall of Macbeth a separate Nemesis Action.
The same reasoning applies to the latter half of the play: the fall of Macbeth not only serves as the retribution for his rise, but further contains in itself a crime and its nemesis complete. What Banquo is to the first half of the play Macduff is to the latter half; the two balance one another as, in the play of Julius Cæsar, Cæsar himself is balanced by Antony; and Macduff comes into prominence upon Banquo's death as Antony upon the fall of Cæsar. Now Macduff, when he finally slays Macbeth, is avenging not only Scotland, but also his own wrongs; and the tyrant's crime against Macduff, with its retribution, just gives unity to the second half of the play, in the way in which the first half was made complete by the association between Macbeth and Banquo, iii. i. 57-72.from their joint encounter with the Witches on to the murder of Banquo as a consequence of the Witches' prediction. Accordingly we find that no sooner has Macbeth, by the appearance of the Ghost at the banquet, realised the turn of fate, than his first thoughts are of Macduff:
iii. iv. 128.
Macbeth. How say'st thou, that Macduff denies his person
At our great bidding?
Lady M. Did you send to him, sir?
Macbeth. I hear it by the way; but I will send.
When the Apparitions bid Macbeth 'beware Macduff,' he answers,
iv. i. 74.