Lady Macbeth fainting removes suspicion from the Macbeth’s in Duncan’s murder.

Macbeth and Lady Macbeth plotted the murder of Duncan and carried it out as he was staying in their house overnight.   Of course it would do no good for people to be suspicious of them, so they take measures to deflect suspicion.

Both Macbeth and Lady Macbeth make a real show of demonstrating how shocked and overcome with emotion they are at the murder of their beloved king.  Their melodramatic acting is designed to fully convince their houseguests that they had nothing to do with it.

Macbeth informs the others that he is covered in blood because he killed Duncan’s guards.  This neatly explains the blood, the knives, and the killing of the witnesses.

Who can be wise, amazed, temperate and furious,(120)

Loyal and neutral, in a moment? No man:

The expedition of my violent love

Outrun the pauser reason. (Act 2, Scene 3)

Lady Macbeth, whose idea this whole thing was to begin with, further plays her part by fainting from the pure horror of what has befallen in her house.  Macduff seems convinced, and asks the others to help her out when she faints.  It certainly seems like she and her husband are above reproach.

The Macbeth’s false grief sharply contrasts with the real pain Macduff, Malcolm, Donalbain and the others are suffering.  The Macbeths try to make it look like they are horrified Duncan fell to injury in their own house, but it is just a little too convenient that Malcolm and Donalbain, the king’s sons, flee and allow Macbeth to take the throne.  It is not long before they begin to suspect that Macbeth and his wife knew what was going on all along, and murdered Duncan themselves so Macbeth could be king.