The primary theme of Macbeth by William Shakespeare is the deadly effects of selfish ambition. While Macbeth at first displays no evidence of this thirst for power, it soon becomes evident that he has an ambition that is going to be deadly for others and eventually himself.
In the beginning of the play, Macbeth is a loyal, faithful, and valorous soldier and kinsman of the king; he is, as far as we know, content in this role--until his encounter with the witches on the heath. They greet him with three titles, one of which he already has, and Macbeth is bewildered at the witches' prophetic words. When the second title is immediately conferred on him, Macbeth is clearly impressed and hopeful that their third prediction, that he will become king, will also come to pass; but he shows no inclination to make things happen out of their natural order. He says:
It is only one short scene later when we begin to see the effects of Macbeth's ambition. He is present as King Duncan of Scotland announces that the successor to the throne will be his oldest son, Malcolm. Clearly Macbeth has done some devious and evil thinking about becoming king; his hunger for power shows in this aside:
The Prince of Cumberland! that is a step
On which I must fall down, or else o'erleap,
For in my way it lies. Stars, hide your fires;
Let not light see my black and deep desires:
The eye wink at the hand; yet let that be,
Which the eye fears, when it is done, to see.
Now he shows his impatience to be what the witches said he would be--and clearly something he has long wanted to be, despite the fact that Duncan has two sons and Macbeth is not in the line of succession. He wants to be king and these "black and deep desires" are beginning to show.
In the next scene, Macbeth goes home to his wife, who learned of the predictions through a letter he sent her, and she displays the same ugly ambition. Even more, she has already formulated a plan to usurp fate and make her husband king by a foul act. When Macbeth tells her that Duncan is coming to visit them tomorrow, Lady Macbeth immediately says:
Shall sun that morrow see!
Your face, my thane, is as a book where men
May read strange matters. To beguile the time,
Look like the time; bear welcome in your eye,
Your hand, your tongue: look like the innocent flower,
But be the serpent under't. He that's coming
Must be provided for: and you shall put
This night's great business into my dispatch;
Which shall to all our nights and days to come
Give solely sovereign sway and masterdom.
This is a clear plan to kill the king while he is visiting their home, and Macbeth does virtually nothing to stop her because, of course, he wants the same thing she does.
The evil ambition the Macbeths demonstrate is costly for everyone in the country. Because he seized power by violence, Macbeth is forced to use violence to keep it. Innocent people are slaughtered and the entire country is in mourning for the loss of what they once had: a good and selfless king.
Both Macbeths suffer from their guilt. Though Macbeth suffers early on and then seems to harden himself to his conscience, the guilt is enough to kill Lady Macbeth. Macbeth loses his life and his soul because he allowed his thirst for power, his greedy ambition to overcome his reason. He commits heinous acts in the pursuit and maintenance of his desire to be king, acts which prove the evil nature of selfish ambition.