First, let's briefly discuss the relationship of murder and the feudal system in the Middle Ages. Noblemen like Macbeth, Banquo, Macduff and others were constantly fighting on behalf of their king--as in the crushing of the rebellion at the beginning of the play--so violence was a major part of their lives, and the idea of murder for political gain, as unnatural as it might seem to us, was an integral part of the feudal system.
Shakespeare's use of muder in Macbeth, however, is meant to show how ambition subverts the natural order of life. For example, Macbeth's murder of Duncan, who is considered a good, if slightly ineffective, king, violates nature and Macbeth's responsibilities: not only does Macbeth kill a king who generally rules well but he aslo kills a kinsman while that kinsman in Macbeth's home, a violation that cannot go unpunished. Nothing good can come of this action.
Murder even subverts the nature of woman. Lady Macbeth, who, as a woman, should be gentle, nurturing, and a gentling influence on her husband, has been completely changed by ambition into a more adamant murderer (or murderess) than Macbeth himself. At least before the killing of Duncan, she views murder as a legitimate way to further her and Macbeth's ambitions.
After Macbeth kills Duncan, the self-perpetuation of murder becomes apparent: he has to kill Banquo, the Macduff and his family. Shakespeare's point, of course, that once on begins murder in a feudal system, one has to continue killing until all the possible opposition is dead, including innocent women and children. Any threat to one's power has to be eliminated, and murder simply continues rolling along because one can't stop until everyone remotely opposed to the initial murder is dead.
Lastly, the fact that Lady Macbeth's guilty conscience essentiallly turns her into a sleep-walking lunatic, attempting to wash non-existent blood spots from her hand and having imaginary conversations with he husband. This is Shakespeare's way of letting us know that guilt associated with murder is inescapable and ultimately destructive.