Lady Macbeth tries to defy gender stereotypes to persuade her husband to murder Duncan. She calls on evil spirits to harden her heart and dry the nurturing milk in her female breasts. She doesn't want any sentimentalism or softness to derail the plan to kill the king and take the throne. She says in act 1, scene 5:
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.
In a reversal of the stereotypical sex roles, in which the man is strong and hard-hearted, the woman weak and soft, Macbeth decides he doesn't want to murder a king who has been so good to him—and he tells his wife so. She then pulls out all the stops to manipulate him into doing the act. She says she would go so far as to dash her baby's brains out if she said she would do so. Macbeth is impressed at her ruthless words and hopes she will be the mother of sons.
Lady Macbeth's ploy to get Macbeth to do her bidding is successful. She challenges Macbeth's masculinity to the point that he goes against his better instincts and murders Duncan rather than be diminished in his wife's eyes as less of a "man" than she is.
On the surface, it seems that Lady Macbeth, from her cruel and ruthless words, is the more "masculine" of the two, defying ideas of women as weak, scared, and nurturing.
However, in the end, Shakespeare does uphold gender stereotypes. For all her fearless words, Lady Macbeth says she can't kill Duncan herself because he reminds her, when asleep, of her father:
Had he not resembled
My father as he slept, I had done’t.
She is, in other words, not such a hard-hearted person as she pretends to be!
Later, too, it is Lady Macbeth who cracks from the guilt of having persuaded her husband to kill Duncan. It is she, not Macbeth, who commits suicide.