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.