What has Macbeth realized about becoming king in Act III, scene 2 of Macbeth?

Macbeth has realized that becoming king and remaining king are two different conditions. In other words, what's done is not done.

When Macbeth says, "If it were done when 'tis done, then 'twere well/It were done quickly" (1.7.1-2) in his soliloquy before he murders Duncan, he believes he can end his dilemma of how to become king. However, he has not considered how to remain king. Now, Macbeth recalls that the prophecy for Banquo has been that his descendants will be kings. So, despite Lady Macbeth's saying "what's done is done" (3.2.12), Macbeth tells her that he suffers from mental torture because there are still threats to the throne from Banquo and his son, Fleance. Further, he says, "there shall be done/A deed of dreadful note" (3.2.45-46), and a worried Lady Macbeth asks, "What's to be done?" (3.1.47).

This repetition of the word "done" is a reminder that nothing is really finished. Macbeth hires murderers to kill his old friend Banquo, along with his son, in order to prevent the witches' prophecy from fulfillment. But this act, too, will not be "done," as Fleance is not killed.