Lady Macbeth's reaction tells us much about her character. After reading the letter, which describes Macbeth's meeting with the witches, their prophecy, and the fulfillment of part of their prophecy when he became Thane of Cawdor. She announces that she fears he lacks the ruthlessness necessary to fulfill the rest of the prediction:
Yet do I fear thy nature;
It is too full o’ the milk of human kindness
To catch the nearest way. Thou wouldst be great;
Art not without ambition, but without
The illness should attend it.
She hopes that her husband will quickly come home, so that she may motivate him to do what she feels must be done:
That I may pour my spirits in thine ear,
And chastise with the valor of my tongue
All that impedes thee from the golden round...
So it becomes evident in this scene that Lady Macbeth will play a crucial role in driving her husband toward fulfilling the witches' prophecy. She will encourage him to overcome whatever moral or ethical objections he has to killing Duncan, in particular. Eventually, Macbeth's actions, and his ambitions, spiral out of control to the point where his wife is no longer the driving force behind him. But at this crucial point in the play, it is clear that his wife will help nudge him toward conspiracy and murder.