Lady Macbeth faints when the death of Duncan is announced by Macduff. We know that she planned the murder. By fainting, Lady Macbeth, does what she thinks she should do. Remember, she asked to be unsexed so she could plan the murder of the king, but since she is a women, she must play the feminine role.
Lady Macbeth also uses the faint to divert attention away from the death of the king and Macbeth's murder of the two guards. Of course Macbeth kills them so they can't be questioned about the murder of Duncan.
Lady Macbeth knows how to be the "innocent flower".
If the two guards had been questioned, they would have been questioned separately. No doubt torture would have been used if considered useful. Both guards would tell the same story. It would surely become evident that they were both innocent. They might both tell about how they felt as if they had been drugged and how they both became unconscious although they had not been feeling sleepy. The guards are simple fellows. Even under torture they couldn't name anyone who had "suborned" them, bribed them, to kill King Duncan, because there hadn't been any such person bribing them. It would soon become evident that someone had drugged them and then that person, or an accomplice, had slipped in later and murdered the King. Who could have drugged them? Well, who gave them their "possets"? They would tell their inquisitors that it was Lady Macbeth. No doubt Macbeth felt he had to kill them when he had the opportunity in order to keep them from being questioned.
Lord and Lady Macbeth planned to pin the King's murder on the two guards. When Macbeth comes back to their bedchamber with the two daggers in Act II, Scene 2, his wife asks:
Why did you bring these daggers from the place?
They must lie there. Go carry them, and smear
The sleepy grooms with blood.
When her unnerved husband refuses, she tells him:
Infirm of purpose!
Give me the daggers. The sleeping and the dead
Are but as pictures; ’tis the eye of childhood
That fears a painted devil. If he do bleed,
I'll gild the faces of the grooms withal,
For it must seem their guilt.
They both must realize that the guards will protest their evidence, but she thinks that their own daggers smeared with blood and the blood on their faces will be sufficient circumstantial evidence against them. However, neither man will be able to explain their motive, even under torture. It would have been bad for Macbeth to let them live, but it was bad to kill them. Macduff already suspects he killed them to shut them up and keep them from naming names. Lady Macbeth pulls a fake faint to prevent Macduff from asking Macbeth further questions and give her poor husband some time to get his story straight.
The message which Lady Macbeth wants to convey by fainting in Act 2, Scene 3, in Macbeth depends on whether we think her reaction is genuine or fake.
If we believe that Lady Macbeth really faints, then she is clearly astonished by Macbeth's previous confession that he killed Duncan's guards. This was not a part of Macbeth and Lady Macbeth's plan, so this shocks Lady Macbeth to the point where she faints.
If we interpret Lady Macbeth's reaction as fake, then we can say that she faints because she wants to create a distraction. In the very scene, before Lady Macbeth faints, Macbeth confesses in front of everyone present that he killed Duncan's guards, whom he blames for Duncan's murder, in anger and out of love for Duncan. However, the guards were important because they could have given an explanation regarding Duncan's murder. This view is shared by Macduff, who asks Macbeth why he killed the guards in such haste:
MACDUFFWherefore did you so?
Lady Macbeth, as calculating and manipulative as she is, realizes that Macbeth may come across as suspicious at that moment, so she decides to shift everyone's attention to herself by fainting:
Help me hence, ho!
Look to the lady...
BANQUOLook to the lady:
LADY MACBETH is carried out