In medieval times, it was believed that the health of a country was directly related to the goodness and moral legitimacy of its king. If the King was good and just, then the nation would have good harvests and good weather. If there was political order, then there would be natural order. Macbeth shows this connection between the political and natural world: when Macbeth disrupts the social and political order by murdering Duncan and usurping the throne, nature goes haywire. Incredible storms rage, the earth tremors, animals go insane and eat each other. The unnatural events of the physical world emphasize the horror of Macbeth and Lady Macbeth's acts, and mirrors the warping of their souls by ambition.
Also note the way that different characters talk about nature in the play. Duncan and Malcolm use nature metaphors when they speak of kingship—they see themselves as gardeners and want to make their realm grow and flower. In contrast, Macbeth and Lady Macbeth either try to hide from nature (wishing the stars would disappear) or to use nature to hide their cruel designs (being the serpent hiding beneath the innocent flower). The implication is that Macbeth and Lady Macbeth, once they've given themselves to the extreme selfishness of ambition, have themselves become unnatural.
Nature and the Unnatural ThemeTracker
Nature and the Unnatural Quotes in Macbeth
That tend on mortal thoughts! unsex me here,
And fill me from the crown to the toe, top-full
Of direst cruelty; make thick my blood,
Stop up the access and passage to remorse,
That no compunctious visitings of nature
Shake my fell purpose, nor keep peace between
The effect and it! Come to my woman's breasts,
And take my milk for gall.
The handle toward my hand? Come, let me clutch thee;
I have thee not, and yet I see thee still.
Art thou not, fatal vision, sensible
To feeling as to sight? or art thou but
A dagger of the mind, a false creation,
Proceeding from the heat-oppressed brain?
I see thee yet, in form as palpable
As this which now I draw.
Macbeth does murder sleep, — the innocent sleep;
Sleep, that knits up the ravell'd sleave of care,
The death of each day's life, sore labor's bath,
Balm of hurt minds, great nature's second course,
Chief nourisher in life's feast.
Creeps in this petty pace from day to day
To the last syllable of recorded time;
And all our yesterdays have lighted fools
The way to dusty death. Out, out, brief candle!
Life's but a walking shadow, a poor player
That struts and frets his hour upon the stage,
And then is heard no more. It is a tale
Told by an idiot, full of sound and fury,