Lady Macbeth and Macbeth certainly fit into the paradoxical motif of "fair is foul, and foul is fair" in Shakespeare's phantasmagoric realm of the preternatural and madness. Doppelgangers of one another, Macbeth and his wife exchange roles early in the drama as the insurmountable and ruthless warrior of Act I, Scene 2 returns to his "dearest partner of greatness" and quickly becomes reluctant to act upon his opportunity to become king because of twinges of conscience. It is then that Lady Macbeth assumes the role of the ruthless, having called upon the spirits to unsex herself so that she may be more masculine, more violent--violence is associated with masculinity in this tragedy--and pursue Macbeth's desire to be king as the witches have foretold.
Ironically, she berates Macbeth,
What beast was't then
That made you break this enterprise to me?
When you durst do it, then you were a man; (1.7.53-55)
and Macbeth admires her "undaunted mettle" that resembles his in the battle against Macdonwald, but when the opportunity to murder King Duncan in his sleep arises, Lady Macbeth becomes weak and sentimental [qualities associated with women in this play] and explains to her husband,
Had he not resembled
My father as he slept, I had done't.
My husband! (2.2.15-17)
In a paradoxical incident in keeping with the fair/foul motif, the once frightened Lady Macbeth resumes her assertive and ruthless role by scolding Macbeth, who fixates upon Duncan's blood on his hands as an "incardine" sign of his guilt,
My hands are of your color, but I shame
To wear a heart so white....
A little water clears us of this deed (2.2.80-84)
As Lady Macbeth grows stronger in her resolve and Macbeth experiences moments of madness, such as when he sees the ghost of Banquo, renowned Shakespearean critic Harold Bloom contends that the bard has Lady Macbeth also go mad from having assumed all their guilt, and die in order that Macbeth cannot be overshadowed by such a strong character in the pursuit of his vaulting ambition. This death of Lady Macbeth also points to the love that does exist between the couple as Macbeth is truly shaken by her death--"She should have died hereafter...."
After the death of Lady Macbeth, "blood will have blood" and the deeper Macbeth's hands are in this blood, the more ruthless and horrified he becomes until he reaches his own mad end imagining that Birnam Wood does, indeed, come to Dunsinane, as predicted by the three sisters. With a reaction much like that of his double, Lady Macbeth, Macbeth contemplates death as an end to his horror,
I ’gin to be aweary of the sun
And wish the estate o’ the world were now undone. (5.5.55-56)
In the end, although Lady Macbeth and Macbeth are doubles, it would seem that the sympathy lies more with Lady Macbeth, who appears more human in her terrible guilt and her pathetic attempts to wash away the blood of their evil deeds as she resumes the role of sensitive woman while Macbeth perverts his masculine role to one of only murderous aggression.