Macbeth seems to have the temperament of a child. He is unable to control his impulses. He also seems unable to avoid the influence of others.
When the witches tell Macbeth that he will someday be king, he seems to take this as golden and decides that he should be king. When he isn’t named Duncan’s successor—and there is no reason he should be—Macbeth basically throws a fit.
Stars, hide your fires;
Let not light see my black and deep desires:
The eye wink at the hand; yet let that be
Which the eye fears, when it is done, to see. (Act 1, scene 5, p. 18)
Whether he wanted to be king before the witches talked to him we do not know. We do know that it did not take much prodding to get him to take the bait.
Macbeth tells his wife about the prophecies, and she pushes him to move on Duncan. He asks her what should happen if they are going to fail. She tells him to be courageous, but he still has concerns.
Will it not be received,
When we have mark'd with blood those sleepy two
Of his own chamber, and used their very daggers,(85)
That they have done't? (Act 1, Scene 7, p. 24)
Once she gets the ball rolling, she is unable to control him though. Macbeth does not follow her influence any more. He continues to act as he wants, and act on a whim. He does not stop until he is killed.