How does Act 5 Scene 1 give insight into the main themes of Macbeth?

Act 5, Scene 1, is the scene were Lady Macbeth sleepwalks and seems to struggle with her guilt over the monster that her husband has become.  She seems to lament Lady Macduff's death, a brutal act about which her husband did not even consult her.  Although Lady Macbeth had everything to do with Duncan's murder, she seems to be struggling now, wondering "who would have thought the old man to have so much blood in him?" (5.1.41-42).  Lady Macbeth seems to be reliving the night on which they murdered Duncan when she, ironically, told Macbeth not to dwell on what they'd done because it would drive them mad.  Now, her guilt seems to be doing just that -- making her crazy -- and so this scene helps to develop the themes of guilt and madness in the play. 

Further, the doctor says that "Unnatural deeds / Do breed unnatural troubles" (5.1.75-76).  In killing a king (while he slept), the Macbeths have committed a crime against nature; even Macbeth was afraid that he would never be able to sleep peacefully again, and now it is Lady Macbeth who cannot sleep peacefully because of their crime.  Thus, this scene helps to develop the theme of unnatural crimes and their effect on those who commit them.