There is never any doubt in Macbeth that Macbeth is driven by his "vaulting ambition." (I.vii.26) However, in striving for his goals, Macbeth is filled with doubt and needs encouragement from the witches and Lady Macbeth in ensuring that he is "more the man." (51)
Lady Macbeth makes it clear from the outset that she will sacrifice everything and the spirits should "unsex me here" (I.v.38) to ensure that her ambition and "fell purpose" (44) are not compromised. She tells Macbeth to "leave all the rest to me" (70) as she knows his resolve may falter.
Shakespeare reveals how Macbeth's ambition is destructive as he finds it easier and easier to kill after each murder whereas Lady Macbeth is driven increasingly mad by her conscience as "a little water"(II.ii.67) may have worked at first, after Duncan's murder but cannot sustain her as she realizes what her husband has become and her efforts to clear her conscience go unrequited - "Out damn spot"(V.i.33) just isn't working for her any more and "more needs she the divine than the physician." (72)
The witches, fully aware that Macbeth is easily manipulated, are able to further their own ambition by making Macbeth, "the wayward son" (III.v.11), rely on their prophesies by "draw(ing) him on to his confusion." (29) Hectate reminds the witches that Macbeth bears "his hopes 'bove wisdom" (31) and by the time he realizes that "be these juggling fiends no more believ'd" (V.viii.19), it is too late and he cannot beat MacDuff.
Thus the audience is left in no doubt that the destructive elements present in Macbeth and Lady Macbeth's characters are enough to ensure that "fair is foul and foul is fair." (I.i.10)