Macbeth is often read as a cautionary tale about the kind of destruction ambition can cause. Macbeth is a man that at first seems content to defend his king and country against treason and rebellion and yet, his desire for power plays a major role in the way he commits the most heinous acts (with the help of his ambitious wife, of course). Once Macbeth has had a taste of power, he seems unable and unwilling to stop killing (men, women, and children alike) in order to secure his position on the throne. Selfishly, Macbeth puts his own desires before the good of his country until he is reduced to a mere shell of a human being. Of course, ambition isn't Macbeth's only problem. Be sure to read about the play's portrayal of "Fate and Free Will" also.
Ambition exists in both good and evil forms in Macbeth. On the one hand, some characters use ambition to act in the best interests of their country. On the other, some characters allow it to take the form of power-lust.
Macbeth portrays excessive ambition as unnatural and dangerous – it can ruin individuals and entire countries.