Ambition, the witches, and Lady Macbeth all influenced Macbeth's decisions in William Shakespeare'sMacbeth.
The witches do play a role in developing Macbeth's thirst for power. After all, he first starts thinking about his advancement after their prophecy that he could be Thane of Cawdor.
The largest contributing factor was Macbeth's personal ambition; the other two factors are external. The witches and Lady Macbeth make suggestions and try to direct Macbeth, but ultimately its Macbeth's own selfish desires for greatness and power that lead him to murder Duncan. In the end, Macbeth must be held responsible for his actions and choices. Ambition is his undoing.
Personal ambition has the biggest impact on Macbeth's decision making process. Macbeth wanted to the King before he even met the witches. He just did not see how that would be possible. After hearing the witches prophecies, he wanted their prophecies to be true. If he did not want to be King, he simply would have said the witches were evil and trying to stir up trouble. However, he does the direct opposite. He begins to act in a way to make those prophecies true. He tries to deflect his actions by implying that these deeds have be ascribed by some higher power, and he is not responsible for his action which is not true.
Lady Macbeth wants to be queen and makes no bones about it. Unlike her husband, she is honest about her ambition. she realizes that she has to use Macbeth as a way to get her crown and does not mind doing so. She pushes Macbeth to see that she is his mirror image and he hers.
The witches are do not tell Macbeth how he will become King. They simply say he will become king. Macbeth just needed a reason to act on the treachery that was already in him, and the witches provided the motivation.