Why does the impetus for Macbeth's quest for power set the tone for rule by an unnatural or illegitimate power source?
In Act 1, scene I, when the witches respond as one: "Fair is foul, and foul is fair," they have set up the premise upon which Macbeth's quest for power that does not, by rights, belong to him, is based. For Macbeth will go against his better instincts, listening to his greed, but also to his wife, Lady Macbeth, whose amorality urges him ever on to his doom. For anyone of Shakespeare's time to upset, or undo, the social order, was punishable by death, and a fitting end for those who would make use not only of treason, but also of witchcraft, to further their aims, would have told the Elizabethan audience immediately that there would be a necessary punishment in store for Macbeth, or for anyone who dared to upend the "natural" order of what had been decreed or fated.
As soon as Macbeth starts meddling with the natural order, he is doomed, in other words. His power will always be "unnatural" and "illegitimate" because he was not born to the role. Many of Shakespeare's characters share a tragic end when they tempt fate: Julius Caesar, Romeo and Juliet, Shylock, in "The Merchant of Venice"; it is a recurring theme for Shakespeare, and any character who is shown to spit in the eye of fate generally doesn't fare well.
It's an interesting conundrum, because Shakespeare reveals himself as both credulous and skeptical at the same time. In many of his plays, you'll see this dichotomy explored in great detail.
The driving force behind Macbeth's quest for power is his raw ambition, i.e., ambition with no moral boundaries guiding it. This type of ambition is unscrupulous and will seek its goal at any cost. Macbeth, with the help of his wife and her own ambitiousness, puts all moral compunctions aside. In Act 1, sc. 7, when he is arguing with himself about whether or not he should kill Duncan, one of his arguments against doing it is the fear of problems that might crop up later. He doesn't mention the fact that it is simply wrong to murder someone. Shakespeare wrote the play in tribute to the new king of England, King James I, who believed that he was divinely appointed to be king. Shakespeare wove this belief into the play by showing how when Macbeth upsets the natural order, God's plan, by killing Duncan, all sorts of terrible things happen to Macbeth. Macbeth is not the divinely chosen king, his ascension to the throne is unnatural, therefore it is doomed. Macbeth's ambition is the impetus to this eventual doom.
Macbeth is a tragic hero, the key word here is hero. As such, the audience must have some feelings for him, some empathy for his struggles. We first meet him as a brave warrior, a strong man, and a loving husband. However, seemingly, the witches, the unnnatural or supernatural, deliver a message that he will be King. In this manner, Macbeth appears to be set on his fatal course of action through a source outside of his own person. As the tragedy unfolds, Macbeth has many good men in Scotland killed, including his best friend, Banquo, and this pattern was all seemingly set in motion by the witches' predictions and reinforced later through the conjuring of the witches' apparitions. The larger question must be: Did not Macbeth use the witches' predictions as an excuse to seize what he wanted, the throne?