In Act 1, Scene 5 of the play Lady Macbeth has received a letter from her husband telling her about the encounter with the three witches, including especially the fact that they predicted he would become king of Scotland. She is ecstatic. However, she has serious concerns about her husband's ability to do what would be necessary for him to become king. The problem, as she sees it, is that he is not wicked enough. He would like to be king. They have talked about it and even speculated a little about how it might happen. But she knows from their conversations, as well as from what she knows intuitively about her husband's character, that he is essentially a good, kind, and loyal person. The only way she can see of getting him to murder the people who stand between him and his succession to the throne would be for her to use all her powers of persuasion in order to make him feel as wicked and treacherous as herself. While reading the letter, she speaks her thoughts aloud:

Glamis thou art, and Cawdor; and shalt be
What thou art promised: 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: what thou wouldst highly,
That wouldst thou holily; wouldst not play false,
And yet wouldst wrongly win: thou'ldst have, great Glamis,
That which cries 'Thus thou must do, if thou have it;
And that which rather thou dost fear to do
Than wishest should be undone.'

Shakespeare did not want to make Macbeth an ordinary villain like Richard III or Iago. He wanted his audience to sympathize with his protagonist and feel some pity for him when he finally died. Characteristically, Macbeth dies courageously in battle. Shakespeare created strong outside forces that imposed on Macbeth and manipulated him into becoming a murderer and then a tyrant. The three weird sisters were one force. They made it seem that what he had to do was inevitable. But the most important force manipulating him was his wife. In the first acts of the play she uses all her powers of persuasion to make him go through with the assassination of Duncan. She is obviously brighter than her husband. She tells him he will lose her love and that he will look like a coward in his and her eyes if he does not act decesively when for the first and probably the only time in their lives they have Duncan sleeping under their roof and at their complete mercy. She even gives him great assistance in committing the deed. She drugs the two grooms and later goes back to smear their faces with blood. She is a complete villainess. Without her, Macbeth would never have killed Duncan and would never have become king.

Macbeth proves that his wife's assessment of his character is correct. It is too full of the milk of human kindness. After he has killed Duncan he is horrified at what he has done. After Duncan's body has been discovered, Macbeth will express his remorse:

Had I but died an hour before this chance,
I had lived a blessed time; for, from this instant,
There 's nothing serious in mortality:
All is but toys: renown and grace is dead;
The wine of life is drawn, and the mere lees
Is left this vault to brag of.   (Act 2, Scene 3)