Lady Macbeth is a strong character at the start of the play, but she gradually descends into madness after she engineers the murder of King Duncan.

Early in the play, Lady Macbeth does not show much weakness. As soon as Macbeth tells her that the witches said he will be king one day, she starts plotting to make it happen immediately. She is a strong wife because she supports her husband's ambitions and will do whatever she deems necessary to help him reach them. When her husband returns from battle, she persuades him to kill Duncan while he stays in their castle. During Act I, while the two are planning the murder, Lady Macbeth is steady, though Macbeth hesitates. Lady Macbeth only acknowledges the weakness that she is a woman, and she wishes to be rid of her feminine qualities and commit the crime herself. In a famous soliloquy in Act I, scene v, Lady Macbeth implores,

Come, you spirits

That tend on mortal thoughts, 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,

That no compunctious visitings of nature

Shake my fell purpose, nor keep peace between

The effect and it! Come to my woman’s breasts,

And take my milk for gall, you murd'ring ministers,

Wherever in your sightless substances

You wait on nature’s mischief.

If Duncan hadn’t reminded me of my father when I saw him sleeping, I would have killed him myself. (II.ii)

Out, damned spot! Out, I say!—One, two. Why, then, ’tis time to do ’t. Hell is murky!—Fie, my lord, fie! A soldier, and afeard? What need we fear who knows it, when none can call our power to account?—Yet who would have thought the old man to have had so much blood in him.

Lady Macbeth is both a strong wife and a weak woman at different points in Macbeth. At the beginning, she is the strong wife who knows that her husband will not be able to decide to kill the king on his own. Before he comes home, she asks the spirits to make her more like a man so she can do what needs to be done. “Come, you spirits that tend on mortal thoughts, unsex me here/ And fill me, from the crown to the toe, top-full/ of direst cruelty (Act I, Scene 5). When her husband expresses his doubts about killing the king, she goads him into changing his mind: “Art thou afear’d/ to be the same in thine own act and valor/ As thou art in desire?” (Act I, Scene 7). Lady Macbeth continues to support her husband and ensure that the murder is successful. “Infirm of purpose!/ Give me the daggers” (Act II, Scene 2). In the aftermath of the murder, Lady Macbeth is by Macbeth’s side to protect him when he is weak. “I pray you, speak not, he grows worse and worse” (Act III, Scene 4).

Lady Macbeth is also weak, although she does not see this in herself until it is too late. During the Duncan's murder, Lady Macbeth experiences a moment of weakness when she looks down at Duncan and sees he resembles her father. She is unable to kill Duncan for this reason. “Had he not resembled/ my father as he slept, I had done ‘t (Act II, Scene 2). Later in the play, Lady Macbeth realizes her guilt is keeping her from enjoying the rewards of the crown. “Nought’s had, all’s spent/ when our desire is got without content“ (Act III, Scene 2). As her husband kills more people and she recognizes that power does not make her as happy as she expected, Lady Macbeth sinks into madness and is unable to recover.