When Lady Macbeth receives a letter from her husband that describes the witches' prophecy for his future, she comments that she fears her husband is "too full of the milk of human kindness" to do what is necessary to fulfill the prophecy. She asks the "spirits that tend on mortal thoughts" to:
...unsex 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!
When Duncan arrives at Macbeth's castle, and the opportunity presents itself to assassinate him, Lady Macbeth pushes her vacillating husband to commit the deed, challenging him to be a man and trying to fortify his courage. She points out that Macbeth has vowed to fulfill his destiny, and goes so far as to say that she would kill a nursing baby to live up to such a vow. She even plans the murder, which will take place while Duncan is sleeping. They will get his two chamberlains drunk and pin the murder on them:
But screw your courage to the sticking place,
And we'll not fail. When Duncan is asleep—
Whereto the rather shall his day's hard journey(70)
Soundly invite him—his two chamberlains
Will I with wine and wassail so convince,
That memory, the warder of the brain,
Shall be a fume and the receipt of reason
A limbec only. When in swinish sleep(75)
Their drenched natures lie as in a death,
What cannot you and I perform upon
The unguarded Duncan? What not put upon
His spongy officers, who shall bear the guilt
Of our great quell?
Later, as she takes the two daggers used to commit the deed, she chastises Macbeth for his remorse, and even plots to smear blood from the daggers on the faces of the two chamberlains. So Lady Macbeth is some ways goads her husband into committing the deed. However, he murders Banquo on his own, and later in the play she shows herself to be wracked with guilt for her husbands actions, as the famous "Out, damn'd spot" soliloquy demonstrates. This show of remorse demonstrates that she is not, like the witches, a pure embodiment of evil. Her remorse comes full circle with her suicide near the end of play.