In Act II, Scene 2 of Macbeth, why is Lady Macbeth not overcome with horror like her husband?
When Lady Macbeth receives her husband's letter describing the witches' prophecy, she say that she fears her husband may not be ruthless enough to do what needs to be done (i.e. the murder of Duncan) to take the throne. So she steels herself to push him toward the act, and to become callous and remorseless:
...[U]nsex me here
And fill me, from the crown to the toe, top-full
Of direst cruelty! Make thick my blood,
Stop up the access and passage to remorse,(45)
That no compunctious visitings of nature
Shake my fell purpose nor keep peace between
The effect and it!
So Lady Macbeth resolves to set aside her conscience as well as, Shakespeare's audiences would have perceived, her femininity. In Act II, Scene 2, with Duncan just murdered, she is as good as her word. She orders him to go and smear Duncan's blood on his bodyguards, and when he is too overcome with revulsion to do it, she does it herself, mocking 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.
Later in the play, Lady Macbeth is no longer able to repress her feelings of guilt, and she goes mad with remorse, as seen in the "Out! damn'd spot!" scene, when she, sleepwalking, compulsively scrubs her hands to get rid of imagined blood. So it seems that she is not completely callous and psychopathic by nature, but is rather attempting to appear stoic and calm in the hopes her husband will as well.