Lady Macbeth goes from being homicidally ambitious to guilt-ridden and trapped in her own mind.
Through the character of Lady Macbeth, Shakespeare demonstrates an arc of guilt. When she first learns that the witches have prophesied promotion and kingship in her husband’s future, she is thrilled. She tells him that if Duncan is not going to name him successor, it is his responsibility to kill Duncan and take the kingship.
Lady Macbeth feels that her husband is too full of “the milk of human kindness” to do what must be done.
Thou wouldst be great;
Art not without ambition, but without
The illness should attend it. (Act 1, Scene V, p. 19)
Lady Macbeth is so sure that her husband can’t do it himself that she plans everything, and makes sure he carries out her plan to the letter. She is ruthless in her ambition, and only falters when she sees that Duncan looks like her father when he lies there sleeping. Still, she makes sure Macbeth fulfills his part.
Shakespeare shows us that Lady Macbeth begins to have her doubts when she can no longer control her husband. Soon he is killing others, and she is no longer in on the plan. He tells her not to trouble herself with his plans.
Be innocent of the knowledge, dearest chuck,(50)
Till thou applaud the deed. (Act 3, Scene II, p. 46)
This is the point where she begins to break down. By the beginning of Act 5, she has totally lost it. She is sleepwalking and never waking, endlessly trying to wash the metaphorical blood from her hands, crying “Out, damned spot!”
Hell is murky. Fie, my lord, fie!
A soldier, and afeard? What need we fear who knows it,
when none can call our power to account? Yet who would
have thought the old man to have had so much blood in(35)
him? (Act 5, Scene I, p. 77)
In the end, the guilt and memory of what she has done is too much and Lady Macbeth kills herself. This is quite a change from the ruthless wife whose ambition cost an innocent man his life.