In Shakespeare's Macbeth, how do Macbeth and Lady Macbeth change roles?

Immediately after Duncan's murder, Macbeth seems overwhelmed by his guilt, while Lady Macbeth seems immune to it.  He begins to panic because he could not, physically, pronounce the word "Amen," and he interprets this as the result of his complete loss of God's blessing.  She tells him, "These deeds must not be thought / After these ways; so, it will make us mad" (2.2.45-46).  Lady Macbeth is afraid that if they dwell on the murder and all the possible repercussions, it will actually drive them insane.  Likewise, Macbeth fears that he will never be able to sleep peacefully again because he murdered Duncan while the king was asleep.  Finally, he claims that there is not enough water in the ocean to cleanse his hands of Duncan's blood: "this my hand will rather / The multitudinous seas incarnadine, / Making the green one red" (2.2.79-81).  However, his wife claims that "A little water clears [them] of this deed" (2.2.86).  She seems utterly without guilt over what they have done.

By the last act of the play, though, Lady Macbeth is seen sleepwalking as a result of her guilty conscience.  Whereas Macbeth once thought that he would not be able to sleep peacefully, it is now she who cannot.  She said then that she only needed a little water to put the deed behind her, but now she says that "All / the perfumes of Arabia will not sweeten this little / hand" (5.1.53-55).  She warned Macbeth not to dwell or else he would go crazy, and it becomes clear that she's been dwelling on the murder and it has driven her mad.  She imagines that Duncan's blood is still on her hands and that she cannot get the "damned spot" out (5.1.37).  Macbeth, on the other hand, has become as ruthless as Lady Macbeth once tried to be, even arranging for the murder of innocent women and children in a show of power.