This is one of the most fundamental question posed in Macbeth, and it yields no easy answers. On the one hand, Macbeth is, in many ways, the victim, or perhaps the subject, of a series of malevolent supernatural interventions. The witches appear before him after the battle, and hail him as king of Scotland, for no reason than to stir up trouble. Later, we learn that Hecate herself is plotting against Macbeth by making the apparitions appear to tell him he can only be killed under what seem like very unlikely circumstances:
Shall raise such artificial sprites
As by the strength of their illusion
Shall draw him on to his confusion.
He shall spurn fate, scorn death, and bear
His hopes ’bove wisdom, grace, and fear.
And you all know security
Is mortals’ chiefest enemy.
So Macbeth is clearly at the mercy, to some extent, of malevolent supernatural forces. Other readers may argue that his wife also bears some responsibility for his actions. When he vacillates before murdering Duncan, she steels him by questioning his masculinity:
When you durst do it, then you were a man;
And, to be more than what you were, you would
Be so much more the man.
Lady Macbeth encourages him to commit the murder, and helps him cover it up afterwards. On the other hand, however, she plays little role in his later actions. He has Banquo and the Macduff family murdered without consulting her, and by the end of the play, while he is consumed by ambition, she is a broken woman.
Ultimately, though Macbeth is to some extent victimized by supernatural forces, he must ultimately bear the responsibility for his actions. His ambition is present from the beginning, and he seems to have little ability to control it, especially after the initial murder.