Lady Macbeth's fainting spell, like everything else she has done so far, has a purpose. What message does she want her fainting spell to convey?
Lady Macbeth's fainting spell intended to make the men think that she was so upset by the news of Duncan's death, that it caused her to faint. She wants to be seen as typical woman would have been viewed - delicate and unable to tolerate harsh, shocking news. She also wants to be seen as ignorant of any information regarding the death of King Duncan. She has to present the appearance of announcement of his death as such a severe shock to her that she could not maintain consciousness. She does not want anyone to suspect her or her husband of the murder. Ironically, Lady Macbeth was more delicate than she thought she was. She invokes the powers of darknessin Act 1 and asks that they remove from her any feminine weaknesses that might prevent her from doing what needed to be done - kill Duncan - to get the crown for her husband. But by the beginning of Act 5, Lady Macbeth has gone mad with guilt. She walks around trying to wash imaginary blood from her hands, mumbling about the evil deeds done by her and her husband, and demanding always that a lighted candle be nearby to dispel the darkness.