The play begins with the witches' prophecy that Macbeth will one day be king. This prophecy plants the seed of ambition in Macbeth's mind, and he begins to plot to make it a reality. He is willing to do anything to achieve his goal, even if it means murdering his own king.
Macbeth's ambition is ultimately his downfall. He becomes so consumed by his desire for power that he loses all sense of morality. He becomes a ruthless tyrant, and his reign is marked by violence and bloodshed.
The tragedy of Macbeth is a cautionary tale about the dangers of ambition. It shows how easily power can corrupt even the best of people.