Lady MacBeth reads the letter from her husband which relates what he has learned from the witches. After she finishes the letter, she reflects upon the character of MacBeth, fearing that his nature is "too full o'th'milk of human kindness" to do what is expedient. While MacBeth has ambition, he is "without the illness should attend it"; he does not possess the coldness and wickedness to take the swift, direct path to the achievement of his goal. Lady MacBeth decides to
pour my spirits in thine ear,
And chastise with the valor of my tongue
All that impedes thee from the golden round
That is, she vows to influence MacBeth with her own wickedness and exigency ("...I feel now/the future in the instant") so that he may earn the crown ("golden round") and become king. These words and Lady MacBeth's speech later in this scene in which she "unsexes" herself by abandoning the feminine characteristics of compassion and tenderness indicate her disturbing change to an evil entity.
Lady Macbeth describes her husband as "too full o' the milk of human kindness/ To catch the nearest way," meaning he is too kind to become king the quickest way, by killing Duncan. He is not ruthless enough, nor is she, for she must call on "spirits/That tend on mortal thoughts ... [to] unsex" her and "fill [her] from the crown to the toe top-full/of direst cruelty." She invokes the powers of the supernatural to fill her with cruelty so that she can assist Macbeth and take advantage of Duncan's impending visit to Inverness. Her request indicates that she is not naturally evil; she must lose her femininity and become tough to achieve their goal.
Right before Macbeth enters, Lady Macbeth apparently has already concocted a plan for Duncan's murder because she refers to her "keen knife" and the "wound it makes." Her next job, then, is to convince Macbeth of what must be done.