This is a fascinating scene in this excellent tragedy, not just because it details the events of the murder of Duncan and those after it, but also because we see Lady Macbeth in a new light. When she enters by herself, we see this resolute and determined woman apparently doubting the success of the plan that she concocted and also showing a surprisingly human side as she explains why she was unable to kill Duncan herself:
Alack! I am afraid they have awak'd,
And 'tis not done:--th'attempt and not the deed
Confounds us.--Hark!--I laid their daggers ready;
He could not miss 'em.--Had he not resembled
My father as he slept, I had done't.--My husband!
However, after this display of fear, doubt and human weakness, Lady Macbeth is quick to reinvigorate her former inexorable nature, which seems to be sparked by her husband's reaction to what he has done. Note the way in which she responds in anger to Macbeth's decision not to go back to the chamber and smear the grooms with Duncan's blood:
Infirm of purpose!
Give me the daggers. The sleeping, and the dead,
Are but as pictures; 'tis the eye of childhood
That fears a pinted devil.
Thus it is that Macbeth's fear and worry and lack of courage seems to inspire his wife to recover from her temporary loss of nerve and to complete the task as was planned, thereby framing the grooms with the murder.