Macbeth is a play of contradictions in every way from the opening scene which states "fair is foul, and foul is fair" to the contrast between the role of fate and free will. Certainly moments in the play, like the scene with the witches' prophesies, suggest that man is nothing but a toy of fate. However, other scenes in the play emphasize Macbeth's personal role in determining his own fate. One such scene occurs as Macbeth debates ending Banquo's life so his progeny cannot interfere with Macbeth's rule as king. He remembers well the witches' prophesy which stated that Banquo's heirs would rule as King, and Macbeth has determined to take action so this event will not occur:
For Banquo's issue have I fil'd my mind,
For them the gracious Duncan have I murther'd,
Put rancors in the vessel of my peace
Only for them, and mine eternal jewel
Given to the common enemy of man,
To make them kings -the seed of Banquo kings!
Rather than so, come, Fate, into the list,
And champion me to the utterance! (III.i.69-76)
In this quote, Macbeth directly challenges Fate, "come, Fate, into the list," as if he wants to duel Fate to determine his own destiny--that Banquo's heirs should not supplant him. Macbeth fully believes that he is writing his own future; he takes it upon himself to plan Banquo's demise.
The play Macbeth is full of signs, prophecies, spirits, and supernatural forces, but ultimately at the end of the day, Macbeth's own decision to embrace his ambition leads to his downfall. Like Banquo, he could have chosen to ignore the witches' prophecies, but instead, he bought into their evil designs and through his belief in the veracity of that proposed future, allowed himself to become a pawn of Fate.