How does Lady Macbeth's character develop throughout the play?

Lady Macbeth is transformed in the course of the play from a self-confident and domineering woman into one who is delusional and suicidal. Through these changes, however, there is still a strange consistency in her character.
Despite the initial forcefulness of her behavior that enables her to browbeat Macbeth, Lady Macbeth is from the start a pathetic, desperate woman. Only someone already pathologically insecure would pray to be "unsexed" in order to have the strength to urge her husband into a wild career of murder. She admits to Macbeth that she would have killed the sleeping Duncan herself had he not seemed to resemble her own father. She's therefore not completely heartless. As Macbeth's behavior becomes increasingly erratic, she begins to panic as well. When Banquo's ghost appears, and she tries to calm Macbeth down by telling him the ghost is merely a hallucination ("the very painting of your fear"), the desperation in her own words is obvious. It is ironic that she eventually begins to imagine things herself, believing that blood is still there no matter how many times it's been washed away.
In the end, Lady Macbeth retreats into a fantasy world of her own, seemingly becoming the opposite of the strong-willed woman she was at the beginning. But all of these successive versions of her personality are manifestations of the same desperate psychosis. It is a transformation governed by a self-destructive will, and in the end, she is reported to have died by her own hand, just before Macbeth goes to his own death in the face of what he concludes is a meaningless universe.