Macbeth is ambitious to become king.  Initially when the witches appear (although he doesn’t openly state that he wants to become king) we can see that he becomes ‘rapt’ with what witches say, showing the stirrings of ambition.  'Rapt' suggests that he is 'enraptured' by the witches - having double connotations of being seduced by their words, and having been enchanted by them.  It is left to the audience's interpretation as to whether the witches simply enhance Macbeth's latent ambitions, or whether they have some more nefarious role in causing him to kill Duncan. Certainly, however, Macbeth’s ambition leads him to interpret things in a positive way for him, even when they are not trustworthy.

Macbeth then enters a crisis between what he wants to gain and what he wants to do.  He wants to be the king, but doesn't want to kill the king to do so.  Macbeth is caught between thinking that ‘chance may crown me without my stir’(that he can sit by and allow fate to work) and that he will have to murder the king.  This is shown in one of his major soliloquies where he says that if this were all he had to do ‘we’d skip the life to come’ - he would give away any possibility of redemption in order to attain the kingship.

Eventually Macbeth is persuaded by Lady Macbeth’s taunts about his masculinity and resolves to kill Duncan.  He is swayed by Lady Macbeth's manipulation, which plays upon his own insecurities.  Thus, Macbeth's ambition is unleashed.  In some ways an audience may initially see Lady Macbeth as more ambitious than Macbeth himself.

Immediately after the murder Macbeth seems to regret his actions, and realise that his ambitions have led him astray, stating that he wishes Duncan could be awoken again.

However, as Macbeth becomes more and more paranoid about being found out, he begins to murder more people.  Moreover, he is never satisfied with what he has, and is caught in a cycle of murder that he quickly cannot control.   Macbeth murders Banquo due to the witches' prophecy that Banquo's children will be kings.  Then he kills Macduff’s family in order to protect his place on the throne.   Despite his initial ambition simply being the throne, therefore, Macbeth is quickly forced to defend his position, leading to further atrocities.  Ambition, as the audience sees, is rarely easily satisfied.

Often Macbeth's ambitions and the feelings surrounding are manifested supernaturally.  Initially, his decision to kill Duncan manifests in the ghostly dagger which appears before him, pointing him towards the king.  Later, his guilt about the actions his ambitions have led him to result in the apparition of Banquo's ghost.

Lady Macbeth is also ambitious, and from the opening of the play worries about her husband’s kindness being their downfall.  Her pure desire for power leads her to shun a traditional feminine role, going so far as ask to become a man (or at least not to be bound by the social expectations of womanhood.)   Lady Macbeth begs to be filled with ‘direst cruelty’, and her ruthless manipulation eventually promps Macbeth to kill Duncan. 

Yet Lady Macbeth's early protestations about her ambitions and her cruelty may be seen as betraying her underlying humanity.  Though Lady Macbeth is undoubtedly a key compulsion in Macbeth's murder of Duncan, she nevertheless displays numerous signs of weakness prior to her eventual suicide.  She notes that she herself could not have killed Duncan due to the fact that he resembled her own father, betraying a key human weakness.  Thus, while initially she argues that they can simply rid themselves of guilt with a 'little water', Lady Macbeth's repressed guilt eventually leads to madness.  In Act 5 of the play, her despair about her actions lead her to subconsciously reveal her guilt in sleepwalking, during which she tries to compulsively wash Duncan's blood from her hands - though she is never able to cleanse herself of this.   Ultimately, Lady Macbeth's inability to rid herself of guilt (the consequence of her ambitions) leads to her suicide.

We may also note that, generally, characters who are good are correspondingly not ambitious.  Macduff, for instance, is tested and shown to be more concerned with supporting a good king than strengthening his own hand.  Similarly, Banquo largely ignores the witches' promises to him, instead correctly identifying them as agents of evil. 

An audience may contextually link Shakespeare's criticism of ambition with several contemporary concerns.  Firstly, as a play centrally warning of the dangers of regicide, ambition and aspiration to the kingship is clearly something to be rejected.  Moreover, we might recourse to the idea of the Great Chain of Being - the belief that every single thing has its place in the world and that it is a sin to aspire to be more than that.  Thus, ambition beyond one's station is a sin.  The effects of such ambition are, as Shakespeare shows, catastrophic, both for oneself and for one's country.