In Macbeth, it is a matter of continuing debate whether Macbeth exercises his own free will, driven by his "vaulting ambition"(I.vii.27) and Lady Macbeth, his "Partner of Greatness"(I.v.9) or whether he is controlled by fate on such "foul and fair a day"(I.iii.38), receiving information from the witches that cause him to believe that it is his destiny to be King and he must commit heinous crimes in order to ensure his fate.
Macbeth is clearly conflicted as is evident when he almost calls off the plan to murder Duncan, recognizing that it is only his ambition which "o'er leaps itself."(I.vii.27) Lady Macbeth's persuasion, as she questions his manliness, something which he has surely proven on the battlefield, is only one element that drives him closer to his ultimate ruin. There are many opportunities for him to allow the natural progression towards being king. He is already Thane of Cawdor so it is not unreasonable to expect further honors in his military pursuit. Banquo has also heard the witches prophesy that his descendants will be king but is not motivated to ensure this prophesy at all cost. Macbeth hears and acknowledges Banquo's warning and yet it still gives him "earnest of success"(I.iii.131) If Macbeth is at the mercy of fate, then he would not need to take any steps at all. It is his free will which single-mindedly compels him.
It is true that he is unstable and unable to assert himself sufficiently when confronted by his wife's unyielding behavior prior to Duncan's murder but that is because of his weak will and self-absorbed personality. Once he has killed Duncan, he does not stop to enjoy his new found position but rather allows neurotic tendencies to overwhelm him. Lady Macbeth, by whom he was apparently so influenced, tries to calm him, assuring him that "a little water clears us of this deed."(II.ii.67) She will in fact be driven to madness by her own guilt and yet Macbeth seems encouraged to murder his friend and stop at nothing.
He is certainly affected by the witches but he is the one who demands to hear that he is invincible and the witches play to his weakness as they recognize when "Something wicked this way comes,"(IV.i.45) meaning Macbeth. If fate is responsible for Macbeth's undoing, he would not seize control the way he does, even believing that "none of woman born"(80) can stop him.
In the beginning, Macbeth is prodded into action by the prophesies of the witches and by his wife. However, during the murder of Duncan, though Lady Macbeth was going to perform the deed, she did not, and Macbeth voluntarily killed Duncan, even though Lady Macbeth had not even asked him to do so. After this murder, Macbeth progressively becomes worse. He chooses to have Banquo assassinated, without any prophesy, and he even lies to Lady Macbeth about his plan. Then, after the "Double, double toil and trouble" encounter with the witches, Macbeth chooses to have MacDuff's innocent family, who poses no threat to Macbeth, killed when MacDuff is not to be found. Though Macbeth is started and pushed somewhat by fate along the path of evil, ultimately, he makes free decisions.