In Macbeth, why doesn't Lady Macbeth trust Macbeth to carry out the plot against the king?
Shakespeare seems to have wanted Macbeth to be somewhat sympathetic rather than an out-and-out villain like his Richard III or his Iago. It is clear to the audience that Macbeth does not really want to murder Duncan but is being forced into it by supernatural circumstances and influenced by others, especially his own wife. Fate seems to have brought Duncan to Macbeth's castle for the first and possibly the only time. Shakespeare makes a point of showing that Duncan has never been there before when Duncan says: "This castle hath a pleasant seat. The air / Nimbly and sweetly recommends itself / Unto our gentle senses." The three Weird Sisters have made a strong impression on Macbeth too, especially when their prediction that he would become Thane of Cawdor came true so quickly. And Lady Macbeth will not give him a minute's peace. She knows he is likely to abandon the whole idea of killing Duncan and knows that he might bungle it even if he agreed to go through with it. She says to herself: "Yet do I fear thy nature; / It is too full o' th' milk of human kindness / To catch the nearest way." Rather than making Macbeth a sympathetic character, Shakespeare has risked making him seem weak and henpecked. He would never have killed Duncan without his wife's insistence and aid. She drugs the grooms. She lays the daggers out for him to use. She has to return the daggers to the king's bed chamber because Macbeth has totally lost his nerve and refuses to go back there. She has to tell him to put on his nightgown. It is her ambition rather than Macbeth's that drives the plot up until Macduff's discovery of the king's body.
In Macbeth, it is clear from the outset who Macbeth's "Partner of Greatness" (I.v.9) is. Macbeth's conflict causes indecision in him and he has already realized that it "cannot be ill; cannot be good"(I.iv.131) when he contemplates what the witches have said. Macbeth writes to Lady Macbeth setting out what the witches have said and that he will be king. Lady Macbeth's own ambition is so strong and although she accepts what Macbeth has told her, "yet I do fear thy nature...is too full of the milk of human kindness"(I.v.13-14) and he may not be able to achieve the position of king in what she perceives to be the easiest or "nearest"(15) way. Accordingly, Lady Macbeth steels herself for action.
Lady Macbeth has recognized the opportunity to advance Macbeth's career and she can see "the future in the instant." (54) She does feel that Macbeth is not taking advantage of the situation and even resembles "the innocent flower."(62) She therefore tells Macbeth to "leave all the rest to me."(70) She will persuade him, after he almost decides against killing Duncan, by questioning his manhood and by convincing him that her plan is solid. After his confusion when he brings the daggers with him, she consoles him as "A little water clears us of this deed."(II.ii.67)
Macbeth is a tragic hero because he is consumed by ambition and spurred into committing murder by Lady Macbeth. After he kills the king, Macbeth is wracked by guilt and paranoia. He is forced to continue committing murders to protect himself from suspicion. Each murder drives him deeper into madness. Macbeth becomes convinced that he will not be beaten when the witches tell him, "none of woman born shall harm Macbeth". When he hears, "Macbeth shall never vanquished be until Great Birnam wood to high Dunsinane hill shall come against him", Macbeth is convinced he is invincible.He believes neither of these things is possible. With each ensuing murder that he commits, Macbeth spirals down toward madness and finally death.
In Macbeth, Lady Macbeth does not trust Macbeth to carry out the plot against the king because she thinks he is too weak and conscientious to commit murder. She knows he has ambition to become king, but he lacks a murderous conscience to kill the king. When she hears the witches predictions, she spurs him toward action. In the predictions of the witches, Lady Macbeth sees a chance to advance Macbeth's career. When Macbeth changes his mind about murdering the king, Lady Macbeth challenges his indecisiveness by questioning his courage and manhood. Lady Macbeth plans to murder the king, but his resemblance to her sleeping father prevents her.