The central theme of Macbeth is ambition and what it costs. Macbeth wins the crown through crime and, at every step, loses sleep, allies, and the meaning of what he has conquered. Around this turn guilt, fear, violence, and the question of how much the witches' prophecy decides what he does.
Ambition appears early. In Act I, scene 7, Macbeth lists the reasons against the crime: he is Duncan's kinsman and subject, and also his host. He says that his only spur is this boundless ambition, and he decides to kill only after Lady Macbeth accuses him of cowardice. On the heath, the witches say he will be king, but they do not order anyone killed. The murder is decided by him and by Lady Macbeth.
The price comes soon. After killing Duncan, Macbeth hears a voice that says he will sleep no more. Now king, he says that treacherous things, once born, only grow stronger, fed by evil, and he orders Banquo killed and then Macduff's family. Lady Macbeth also pays: in Act V she walks in her sleep, rubbing her hands.
In the end, Macbeth has the throne and none of what he expected from it. He says that respect, love, obedience, and friends are forbidden for him to desire (Act V, scene 3), and when he learns of the queen's death, he describes life as a tale told by an idiot, full of sound and fury and empty of meaning.