How is it true that Macbeth has been read as a "tragedy of ambition"?
Macbeth and Lady Macbeth are truly led to tragic ends by their ambition.
Seduced by the witches' prophesies, Macbeth fails to listen to the wise advice of Banquo who tells him that the "instruments of darkness" (1.3.133) often tell men part of the truth in order to lead them to their destruction: "to betray's in deepest consequence" (1.3.134). Instead, Macbeth is admittedly driven by his desire to be king because of his "vaulting ambition" (1.7.27), and he is eventually destroyed as he allows his fears and imaginings to "o'erleap" themselves and carry him to his demise. For in these wild imaginings, he commits murder after murder in order to remove anyone that he feels threatens his ambitions for absolute power.
Likewise, Lady Macbeth is destroyed by ambition for her husband's success. For instance, after she reads the letter informing her of the prophesies of the three witches, she becomes so ambitious that she wants Macbeth to hurry home so that she can persuade her husband to ignore whatever is deterring him from seeking the crown. Moreover, she asks the spirits to "unsex" her and fill her with "direst cruelty" (1.5) so that she can aid her husband in his ambitions. This course of action becomes tragic as she, too, is eventually destroyed by an ambition that spirals out of control. Lady Macbeth goes mad with guilt in her reliving the murder of Duncan over and over.
Clearly, Macbeth is destroyed by his ambition to become King of Scotland. Once the witches' tempt him with their prophecies, he cannot turn back. After he and Lady Macbeth begin to plan King Duncan's murder, Macbeth tries to stop their conspiracy by telling her they won't discuss it any further, but his resolve does not last very long. He kills Duncan for one reason only: to gain the crown. Macbeth recognizes his own "vaulting ambition," and he knows killing the innocent, defenseless Duncan as he sleeps will be a most horrible act, but he proceeds.
So wherein lies the tragedy? Like Shakespeare's other tragic heroes, Macbeth had lived an honorable life. Before his ambition conquered his conscience, Macbeth had been a brave and loyal soldier, one who had been trusted and admired by King Duncan. Macbeth's tragedy is that he destroys himself because there is a flaw in his character: His ambition is stronger than his morality.