Lady Macbeth has to steel herself so she can goad Macbeth into murdering Duncan. She knows her husband well, for they are very close as the play opens, and she can sense he will backtrack on the plan for the assassination. Therefore, she calls on the gods to harden her and turn her mother's milk of human kindness into gall (a bitter substance). She associates her compassionate side with femininity, and specifically with the maternal instinct. To her, a mother's nurturing milk represents exactly the loving instincts she wants to quash.

Lady Macbeth associates the shedding of blood with masculinity, not surprising in the warrior culture she is part of. After all, her husband has recently cleaved the traitor Macdonwald in half to protect the crown. Men are taught to shed blood in this culture.

Lady Macbeth wants to bring out Macbeth's more violent, bloodthirsty instincts, which she connects to his masculinity. Ironically, for all her brave talk, she cannot kill Duncan because she thinks he looks too much like her father. She is also the one who cracks later under the guilt of all the bloodshed—ironic, because when Macbeth comes back after having murdered Duncan filled with horror at what seemed like the oceans of blood he shed, she is the one who tells him to shake it off. Yet she is one who ultimately commits suicide because she cannot bear the guilt. It seems, therefore, that it is not only Lady Macbeth but Shakespeare too who sees women as filled more than men—even a woman like Lady Macbeth—with the "milk" of human compassion.