Why is the theme of the breakdown of the natural order explored by Shakespeare in Macbeth?

In Shakespeare's Macbeth, the breakdown of the natural order reflects society's belief in the Great Chain of Being, defined below:

Its major premise was that every existing thing in the universe had its "place" in a divinely planned hierarchical order, which was pictured as a chain vertically extended.

In other words, God ("divinely") placed everything in an order of importance. God was at the top, the angels were beneath Him and the king was next. Below the king would be noblemen of various, descending ranks, and then commoners, etc. All things in the universe (they believed) had a proper place. For instance, the lion was the most important animal; gold was the most important metal; the rose was the highest flower; and so on.

The Elizabethan society believed whole-heartedly in this order, much the same way that they believed in witches and fairies. In keeping with these beliefs, the people of Shakespeare's time lived with an unwavering belief that all would be well in the universe as long as everything stayed in place. Disruption of order is a major theme in the play.

The natural order is inverted as unnatural forces (e.g. murder, the witches, betrayal) bring chaos to Scotland.

In Macbeth, Shakespeare capitalizes on this belief. He draws his audience in with their understanding of an ordered universe. God ordained who would be king—for this reason it was a mortal sin to commit regicide. (The theme of disruption of order is present in quite a number of Shakespeare's plays: such as , , etc.) When Macbeth murders Duncan and takes his place on the throne, the universe is thrown into turmoil. This is reflected, for example, in the strange things people witness and report in Act Two, Scene four. First, Ross reports that there is an eclipse:

By the clock ’tis day,
And yet dark night strangles the travelling lamp. (7-8)

The Old Man shares his observations. (Note that when he speaks of the unnatural "deed that's done," he is referring to Duncan's murder.)

’Tis unnatural,
Even like the deed that's done. On Tuesday last
A falcon towering in her pride of place
Was by a mousing owl hawk'dat and kill'd. (12-15)

The disruption of order is demonstrated, in this case, when a [smaller, less powerful] mousing owl attacks and kills a falcon. The falcon is the stronger bird; mousing owls hunt mice, not larger birds.

ROSS:

And Duncan's horses—a thing most strange and
certain—
Beauteous and swift, the minions of their race,
Turn'd wild in nature, broke their stalls, flung out,
Contending ’gainst obedience, as they would make
War with mankind. (16-21)

Ross describes how Duncan's horses became wild and aggressive toward people, exhibiting unnatural behavior. Worse than this, however, the Old Man reports:

’Tis said they eat each other. (22)

There are further disruptions when Macbeth has Banquo killed—Banquo's ghost appears at the banquet to torment the man responsible. It takes place again when Lady Macbeth begins to walk in her sleep and recalls aloud (and acts out) the circumstances surrounding Duncan's murder. The actions of the witches also cause a disruption because Macbeth is in league with the devil—of this, the audience was sure! 

In Macbeth, Shakespeare uses these unnatural occurrences to support the theme of the disruption to the Great Chain of Being. It reinforces that Macbeth has changed who will rule Scotland—that his tragic flaw (vaulting ambition) has driven him to kill Duncan. He has also flown in the face of God by removing His ordained servant from the throne. This imbalance in nature and in heaven will only be repaired when Macbeth is removed (killed) and the rightful king (in this case, Malcolm) is returned to the Scottish throne.

In including elements of the disruption to nature, the play was more exciting for those watching because it was much more believable.