There would have been no plot of murder if Macbeth had not become overly ambitious from the witches' prophecy. In the beginning, he seems to be satisfied as a good soldier whose sword steamed with the enemy's blood:
Because brave Macbeth, well he deserves that name,
Outshining fortune, with his brandished sword,
Which was steaming with blood in the hot use of it,
Like power’s hero,
On the battlefield, side by side with Banquo, Macbeth seemed to carry out his job with no signs of being overly ambitious. In fact when he met with King Duncan, he admitted his loyalty to King Duncan:
In doing the service and the loyalty I owe you,
I am well paid. Your highness' role as King
Is to receive our duties. and our duties
Are to your throne and state, children and servants,
Who only do what they should, by doing everything
Loyal to your love and honor.
Clearly, Macbeth is quite contented to serve King Duncan at this point.
Then, when he meets the witches, the weird sisters, things change. Macbeth begins thinking about being king of Scotland. Indeed, the weird sisters planted devilish ideas in Macbeth. Becoming Thane of Cawdor only stirred up Macbeth's ambition. Seeing that prophecy come to past only motivated Macbeth to begin planning King Duncan's murder. Immediately, he sends a letter to his wife. He is excited about the prospect of becoming king. In his letter to Lady Macbeth, Macbeth shares how he met the witches and what they did say to him:
"They met me on the day we won the battle, and I have
learned by the most perfect report that they have more in them than mortal knowledge. When I burned in desire to question them further, they made themselves vanish into thin air. While I stood captivated in the wonder of it all letters came from the king, who all-hailed me, 'Baron of Cawdor'; by which title, these weird sisters had just saluted me, and referred me to the future, with 'Hail, king that shall be!'
From this letter written by Macbeth, we can see how he burns with desire at the prophecy. He "stood captivated in the wonder of it all." Macbeth is filled with ambition. He is forever changed. The prophecy will come to past at all cost.
In Shakespeare's Macbeth, when the audience first meets Macbeth, he is just coming from the battlefield with his close friend Banquo. Both men have fought valiantly against Norway, for Scotland, and their prowess in battle has been shared with King Duncan. Up to this point in Macbeth's life, we can assume that he done all for his King (also his cousin and his friend) and his country. However, when the Weird Sisters (witches) meet Macbeth on the heath (and they know he is coming), they are intent upon twisting his allegiance away from serving his country to become self-serving. The witches call Macbeth by three names: Glamis, Cawdor, and he who shall be King. He is already the Thane (earl) of Glamis; upon meeting the King's messenger, Macbeth learns that he has been given the title (and all that comes with it) of the former Thane of Cawdor (a traitor to Scotland). When this happens, Macbeth starts to believe he might well be King of Scotland as well; however, to do so, he takes it upon himself to guarantee this...ultimately by killing Duncan, although he respects and loves him.
By the end of Act One, Macbeth is honest enough with himself to know exactly where his allegiance now lies. According to Aristotle's definition of a tragic hero, Macbeth meets all three requirements: he is a great man, a valiant warrior; he will die; and, his death will be his own fault brought on by his "vaulting ambition," or ambition that knows no bounds—ambition that will drive him to commit a mortal sin: killing a king.
I have no spur
To prick the sides of my intent, but only
Vaulting ambition, which o'erleaps itself,
And falls on th'other. . . . (I.vii.25-28)
Once Macbeth (with the encouragement of his wife, Lady Macbeth) kills Duncan, and Duncan's heirs flee Scotland for fear of their lives, Macbeth becomes King and then goes about making sure that nothing stands in his way of remaining Scotland's monarch. He even kills Banquo (who the witches say will father a line of kings) and attempts to kill Banquo's son Fleance, in order to make sure that none of Banquo's "issue" will take the throne after him. He realizes too late that he has sold his soul for a "barren sceptre" for he has no living heirs. In Act III, scene one, Macbeth faces the reality of his situation—knowing he must kill Banquo and Fleance:
[The witches] hail'd [Banquo] father to a line of kings:
Upon my head they placed a fruitless crown
And put a barren sceptre in my gripe,
Thence to be wrench'd with an unlineal hand,
No son of mine succeeding. If't be so,
For Banquo's issue have I filed my mind,
For them the gracious Duncan have I murdered (64-70)
From the moment the witches plant the idea that Macbeth will be king, he makes sure, driven by his own ambition, to make sure that the prophecy comes true.