Before he commits the murder of Duncan, Macbeth seems to realize something that Lady Macbeth does not. He says, "If it were done when 'tis done, then 'twere well / It were done quickly" (act 1, scene 7, lines 1–2). In other words, if he could simply become king, secure in his power and position, only by committing this one terrible act, then it wouldn't be so bad to just do it and get it over with so that he and his wife could begin this next chapter. However, Macbeth seems to understand that it will not be enough to just kill the king; more will ultimately be required of him and his wife in order to maintain their new privilege.
To be fair, Macbeth seems never to have told Lady Macbeth about the "weird sisters'" prophecy for Banquo, that he would father kings, and so she is, perhaps, unaware of any reason that Macbeth would feel the need to get rid of his one-time best friend. Later, still, Macbeth tells Lady Macbeth, "We are yet but young in deed" (act 3, scene 4, line 176). Lady Macbeth has long thought of her husband as somewhat weak, implying that he is not a man if he will not kill Duncan or if he cannot stop hallucinating. Thus, Macbeth arranging for the murder of Banquo, the attempted murder of Fleance, and the horrifyingly unnecessary murders of the wife and children of Macduff must come as a shock to her. She has helped to create a monster, so to speak, and the extent of the evil to which Macbeth is driven seems to weigh heavily on her conscience. When she sleepwalks in act 5, scene 1, she asks,
The Thane of Fife had a wife, Where is
—What, will these hands ne'er be clean?
(act 5, scene 1, lines 44–45)
Thus, she links the deaths Macbeth has caused to her own guilty conscience, and this seems to be a major factor in their changing relationship. She had evidently been prepared for one murder but not for multiple—and certainly not for the murders of a woman and children who had done nothing wrong.
Her developing understanding of the monster she created drives Lady Macbeth and Macbeth apart. He is prepared to go to much greater lengths than she is to retain their power, and his conscience is better able to support these crimes than hers is because he was more prescient than she was prior to the first one.