What has happened to Lady Macbetch? What is the main cause for her behaviour? Act 5 Scene 1
Well, one thing is for sure; Lady Macbeth was not "unsex[ed]" by the "spirits that tend on mortal thoughts." If you recall in Act I, scene v before Macbeth comes home and before they murder King Duncan, Lady Macbeth asks the "spirits that tend on mortal thoughts" to "unsex" and to "Make thick [her] blood; / Stop up th' access and passage to remorse." In other words, she doesn't want to be human anymore so that she will not be bothered by her conscience after they murder the king.
Well, in the act to which you refer, it reveals that her conscience is bothering her immensely, so much that it manifests itself in her subconscious causing her to sleepwalk, and while she is sleepwalking, she is attempting to cleanse herself of the evil deeds that she has been apart of, in what seems to be a confession, for in it she mentions Duncan's, Banquo's and Lady Macduff's murders. But it doesn't work, for later in the Act she commits suicide.
This scene is extremely ironic when you take into consideration her earlier role in Act I.
In this scene, a doctor is talking with Lady Macbeth's servant. The servant reports that Lady Macbeth has been sleepwalking. While they are talking, Lady Macbeth begins to walk and talk in her sleep. She utters one of her famous speeches, "Out, out damned spot!" In her sleep, she is trying to wash blood off of her hands, the blood of the murdered Banquo. Lady Macbeth has gone mad. Her guilty consience over her part in the murder has brought her down from a strong-willed, plotting woman to a guilt-ridden madwoman. The doctor discerns that she is disturbed and that there is not much he can do to help: "More needs she the divine than the physician" he mumbles. "Unnatural deeds do breed unnatural troubles."
As in many Shakespearean tragedies, this scene reflects the severe consequences of one's evil acts. Shakespeare's audience would be well aware that nothing remotely good could come to Macbeth or Lady Macbeth because of their horrible crimes, because they usurped natural law (their "unnatural deeds"). While her husband's strong character fights to the death, Lady Macbeth's weak character crumbles easily as a result of what they have done. Shakespeare is making the point that evil actions have severe consequences. The actions of Macbeth & Lady Macbeth are straightforwardly evil; thus, so too are the consequences.