In addition to power and dominance, the first person pronoun “my” is used to underscore the important role that Lady Macbeth has in the play, and demonstrate her equal strength.
Sometimes Lady Macbeth uses the first person pronoun to tie herself to Macbeth.
Your face, my Thane, is as a book where men
May read strange matters. (Act 1, Scene 5, enotes etext p. 20)
When she calls him “my Thane” she is both flattering him and pushing him to her will. She goes on here to counsel him to be nice to Duncan so he can more easily kill him. When she calls him “my Thane” or “my lord” she is manipulating him, trying to get him to do what she wants.
In one of her most grisly speeches, Lady Macbeth tells her husband that she has been a mother, and she would willingly bash her newborn baby’s brains out to get what she wants.
I have given suck, and know(60)
How tender ’tis to love the babe that milks me:
I would, while it was smiling in my face,
Have pluck'd my nipple from his boneless gums,
And dash'd the brains out… (Act 1, scene 7, p. 24)
As gruesome as the speech is, it demonstrates that she is strong and equal to Macbeth. He needs to be a killer, and she would be a killer herself. She also says, “What cannot you and I perform upon
The unguarded Duncan?” (p. 24), including herself in the package. She even indicates that she would kill him herself, if he did not look like her father (p. 28-9).
Another example of Lady Macbeth’s complicity in the crime is when she takes the knives from him.
My hands are of your color, but I shame(80)
To wear a heart so white. (Act 2, Scene 3, p. 31)
She is acknowledging that she is nervous, but she has equal blame in the murder. At the end, in her famous sleepwalking speech, she declares “what’s done cannot be undone” (p. 78). She once again takes equal credit, and this time equal blame, for her part.