How does greed play a role in "Macbeth"?
The word "greed" doesn't actually appear in the play itself: though that doesn't mean it's irrelevant to "Macbeth" and its themes. More usually you'll come across critics and scholars talking about the theme of ambition in the play, rather than greed.
Macbeth has "black and deep desires", he admits in his first soliloquy, which seem to have much in common with the witches' prophecies: clearly he has considered already the idea of being king. Why is Macbeth ambitious and greedy for power - and when did it start? Shakespeare never answers that. Yet he calls Lady Macbeth in the letter he sends her his "dearest partner of greatness", and - as everyone knows! - she too holds ambitions for her husband and the "golden round".
If Macbeth is greedy, he is greedy for power, greedy for the crown. And, once crowned, the Macbeths are still not secure, because (as Macbeth says) he only holds a "barren sceptre" in his grip: he doesn't have any children. To keep the crown in the family, Macbeth must ensure the succession, and (as Elizabethan audiences would remember of the recently-deceased Elizabeth I's reign) succession is crucial to maintaining rule.
And then Banquo's death, Fleance's escape - and a number of other deaths pile up one after the other. And all because of one man's ambition fuelled by some supernatural solicitings.