Elizabethans regarded the crime of regicide--killing a king--as a terrible thing, so terrible in fact that nature itself would revolt against it and reveal its discontent in strange signs and unpredictable happenings. This is something that can be seen in this scene directly before the discovery of the death of Duncan, when Lennox and Macbeth are talking together. Note what Lennox says about the night:
The night has been unruly: where we lay,
Our chimneys were blown down; and, as they say,
Lamentings heard i'th'air; strange screams of death,
And prophesying with accents terrible
Of dire combustion, and confus'd events,
New hatch'd to th'woeful time, the obscure bird
Clamour'd the livelong night: some say, the earth
Was feverous, and did shake.
Note Macbeth's rather curt response: "'Twas a rough night," which seems to cover up his guilt at what he knows to be the real reason behind these portents. Clearly, something terrible has happened in the murder of Duncan, and nature itself is raging against Macbeth's crime.
The natural, the supernatural and the unnatural are significant issues in Macbeth. When the play starts, there is a storm and the audience is introduced to the witches who, according to Banquo, "look like the inhabitants of the earth" (I.iii.41) but who are clearly not natural, with beards that belie their appearance as women. Macbeth is so overwhelmed by the witches talk of being king that his thoughts go against "the use of nature" (137). Lady Macbeth will have overriding concerns that the "compunctious visitings of nature" (I.v.42) will stop her from ensuring her purpose to the point of wishing for a most unnatural act as she wishes the spirits to "unsex" (38) her so that the natural order of things does not interrupt her intentions.
The thread continues as Banquo is unable to sleep, presumably because he is somewhat disturbed by the witches prophecies and their apparent effect on him and his friend, Macbeth. Banquo thinks that if he sleeps, he will have nightmares because of "cursed thoughts that nature gives way to in repose" (II.i.8). Banquo recognizes that the natural order of things is off balance, although he cannot predict what may result. Macbeth, also deeply affected by the unnatural events that surround him, knows that the dagger which clouds his judgment is a figment of his imagination and yet the image, to him, has far-reaching consequences as even "Nature seems dead..."(50). Macbeth's obsession with having "murder[ed] sleep," (II.ii.36) which he calls "nature's second course, Chief nourisher in life's feast" (39), makes it clear that Macbeth's attempts to usurp the natural order of things has not worked.
The audience is in no doubt, by Act II, scene iii that an "act against nature" has therefore taken place and the witches have had a part in that by feeding Macbeth's "vaulting ambition" (I.vii.27). Discussions take place regarding the unusual weather conditions of the night before; they were "unruly" with "strange screams of death" (54), and the audience prepares for what it knows will follow. Duncan's body is discovered and Macbeth relates what he saw: Duncan's "gashed stabs looked like a breach in nature" (112). Macbeth is a soldier and a breach, in the military sense, would be a serious problem for an army, meaning that their defenses are compromised and that the enemy is advancing. Therefore, a breach against nature suggests far more than the death of one man but the potential for much greater harm against mankind.