Could someone please explain the theme of ambition in "Macbeth"?
Shakespeare links ambitions (especially ambition for power) with evil and eventually with death. Macbeth is so determined to reach his "rightful" place, he will do anything to achieve it, at his wife's urgings. Lady Macbeth's ambition allows her to push her husband into doing deeds they would have never considered before hearing the prophecy. These acts (beginning with the killing of Duncan) lead Macbeth and his wife to their eventual deaths.
Ambitions also turns the Macbeths into monsters. Macbeth kills his "boss" and the ruler of his country, kills one of his best friends, and becomes so desensitized to death that he barely cares when his wife dies, instead becoming depressed and musing about the futility of life.
In the beginning of the play, Mcbeth's ambition was to live a life of honor, which to him (and all others ) meant both in terms of character and in terms of society. By living an honorable life, one would be given honors. Both meanings were inseparable. Yet, with the the Three Weird Sisters' prophecy, Macbeth severed them one from the other. Macbeth's new ambition was to gain the honors promised him, even if he had to be dishonorable to attain them. His hubris (pride) convinced him that his character was above honor, that anything he did to achieve the honors propehsied, would reflect back and make him honorable again, simply by achieving his ambition to be king.