How is there any free will at all in Macbeth?
Macbeth (and Banquo) receives the initial prophecy from the witches, but the prophecy only says that he will be Thane of Cawdor and king. It does not say that he will murder the sitting king, or predict any of his actions. It is Macbeth and his wife who determine to kill Duncan. Similarly, Macbeth determines to murder Banquo, again based on the witches' prophecy that his descendents willl be kings in the future. There is no doubt that the witches are intervening in Macbeth's life, and that they mean to do harm. Hecate makes that particularly clear in her speech in Act III, Scene 5 (a scene which was admittedly not in Shakespeare's original version):
[I] 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.
The witches, then, conjure spirits and make prophecies that play on Macbeth's considerable ambition, as well as on his insecurities. But no one (except, arguably, his wife) forces him to do the things he does to secure the crown. He decides what course of action to take, acting from his own will to gain more power. This analysis shows that, yes, there is free will demonstrated in Macbeth.
It should be noted that Macbeth specifically does not want to kill Duncan. He is acting against his will. In that long soliloquy in Act 1, Scene 7, he gives all the reasons why he should not commit the murder, and then he tells his wife in the same scene:
We will proceed no further in this business.
He hath honored me of late, and I have bought
Golden opinions from all sorts of people,
Which would be worn now in their newest gloss,
Not cast aside so soon.
Even after he commits the murder he is horrified by what he has done. At the end of Act 2, Scene 3, he says:
Wake Duncan with thy knocking. I would thou couldst.
Shakespeare apparently wanted Macbeth to be a sympathetic character, a tragic hero with only one fault, and tried to invent many excuses for his conduct. Even when he goes off to commit the murder he is in a sort of hypnotic state and is being led towards the sleeping King by a mysterious floating dagger. The murder of Duncan seems to me a prime example of a man trying to exert free will and not being able to do so.