When we first meet Lady Macbeth, she seems to be a relatively conventional woman and wife of the medieval noble class: she appears to be completely supportive of her husband, Macbeth, and she is both attractive and graciousness. But we also see a streak of ambition in her that foreshadows her later actions--she worries, for example, that Macbeth isn't strong enough to be king because he is "too full o' the milk of human kindness," that is, much too kind to be a strong king.
As her ambition for Macbeth takes shape, Lady Macbeth realizes that she will have to supply the force of personality that her husband lacks when she says "make thick my blood; Stop up the access and passage to remorse. . .that my keen knife see not the wound it makes." At this point, clearly, Lady Macbeth is no longer the gracious, attractive wife; she's transformed herself into an ambitious killer in order to convince her less-ambitious husband to kill not only his king but his kinsman.
Later, Lady Macbeth--having asked the witches to "unsex" her (make her a man)--in a complete reversal of sexual roles, she actually accuses Macbeth of acting like a woman: "When you durst do it, then you were a man." In other words, when you finally kill King Duncan, then you'll be acting like a man." Although critics of the play have argued about who was the more ambitious--Lady Macbeth or Macbeth himself--there is no doubt that Lady Macbeth pushes Macbeth to the edge of regicide (killing his king).
Ultimately, however, Lady Macbeth's softer side asserts itself when, after Macbeth is king, she begins to sleepwalk, trying to wash an imaginary blood spot from her hands, clearly feeling a more normal sense of guilt for her part in the killing. This is consistent with the medieval view of women as generally weaker than men and, more important, the view that one cannot adopt unnatural behavior and not suffer for it.