Whatever might be wrong with Macbeth and Lady Macbeth, there is clearly some kind of mental disorder there. They might have been driven by guilt, but there was some madness before. The madness drives them, but also destroys them.
Macbeth first shows signs of mental illness when he sees the floating dagger. He wonders why he can see it but not touch it, and thinks it’s a figment of his “heat-oppressed brain” (p. 27) but it might be real. In the end, the vision seems to push him on to actually kill Duncan.
Whiles I threat, he lives;
Words to the heat of deeds too cold breath gives. (Act 2, Scene 2, enotes pdf p. 28)
When he hears the bell, he does the deed. Through the vision of the dagger, he has talked himself into the act.
Macbeth’s second hallucination occurs after he has killed Banquo, formerly his friend and fellow soldier. Macbeth sits down to dinner with his guests, and suddenly “the table’s full” (p. 50) because Banquo’s ghost is sitting in his seat! If Macbeth is frightened at first, he soon becomes angry.
Why, what care I? If thou canst nod, speak too.
If charnel houses and our graves must send(85)
Those that we bury back, our monuments
Shall be the maws of kites. (Act 3, Scene 4, p. 50)
Lady Macbeth is frustrated, and tries to make excuses, but soon Macbeth is acting so strangely that everyone has to leave. Macbeth seems to really be succumbing to guilt and madness since killing Duncan and having murders kill Banquo and Fleance. He also has the entire Macduff family killed, while Macduff is away. This mistake eventually costs him his life.
Lady Macbeth maintains her sanity until the last act. By this point, she realizes that she is partly to blame for the bloodshed and what has happened to her country. She begins to sleepwalk and hallucinate that there is blood on her hands that she cannot wash off.
Out, damned spot! Out, I say! One–two—
why then ’tis time to do't. Hell is murky. Fie, my lord, fie! (Act 5, Scene I, p. 77)
Lady Macbeth is surprised that Duncan could “have had so much blood in him” (p. 77). When Macbeth hears of his wife’s death, apparently by suicide, his reaction is even stranger. He says she “should have died hereafter” (p. 84) and gives a speech about life that demonstrates his own feeling of mortality.
This is only a brief moment of mental clarity though. Macbeth gets so caught up by the witches predictions that he thinks Macduff cannot kill him. When he finds out that Macduff was “untimely ripp’d” from his mother’s womb by c-section, he gives up.
Accursed be that tongue that tells me so,
For it hath cow'd my better part of man!
....keep the word of promise to our ear,(25)
And break it to our hope. I'll not fight with thee. (Act 5, Scene 8, p. 88)
Macduff is shocked by this reaction and calls Macbeth a coward, then chops his head off and rids the world of the scourge of Macbeth.