At the end of act 3, scene 4, Macbeth feels that he is very much the victim of fate. His feast has just been disturbed by the ghost of Banquo and, even before this, the murderers have informed him of their failure to kill Fleance. This escape lends weight to the notion that Fleance is fated to be King of Scotland, though one might argue that the cause lies in Banquo's exercise of free will. He chose to send Fleance away and face the murderers alone.
At this point in the play, however, Macbeth, demonstrates that, far from being the plaything of fate, he continues to exercise agency in maintaining his tyranny. When he says that Macduff has been avoiding him intentionally, and Lady Macbeth asks how he knows this, Macbeth replies:
I hear it by the way; but I will send:
There's not a one of them but in his house
I keep a servant fee'd. I will to-morrow,
And betimes I will, to the weird sisters:
More shall they speak; for now I am bent to know,
By the worst means, the worst. For mine own good,
All causes shall give way: I am in blood
Stepp'd in so far that, should I wade no more,
Returning were as tedious as go o'er...
There are several points here that pertain to Macbeth's free will. The first is that he has, of his own volition, established a network of spies to report to him on the words and deeds of all his thanes. This appears to be entirely his own idea. He then says that he will go and seek further advice from the witches. They have not summoned him. He will go to them and force them to speak. Macbeth then says that every other consideration must take second place to his "own good," a definitive statement of self-interest and self-determination. He ends by saying, not that he is fated to go on killing, but that it would be "tedious" to try being virtuous now. In this fairly short passage, therefore, there are four separate indications that Macbeth acts of his own volition.
Fate is defined as events outside of a person's control, regarded as determined by a supernatural power. Free will is a person's choice regarding the events of his or her own life, such as what career path to follow.
In Shakespeare's Macbeth, the title character's fate is prophesied by the three witches. They tell him he will be Thane of Cawdor and then king. He sets about making this fate come true through acts of his own free will.
In act 3, Macbeth has become king by murdering Duncan. He is now bothered by the prophecy of the witches that his descendants will not sit on the throne but instead Banquo's will. Since this knowledge torments him, he once again tries to intervene with fate by devising a plan to kill Banquo and Fleance.
Unfortunately for Macbeth, his attempts at bending fate fail, and the prophecy stands when Fleance escapes the murderers appointed by Macbeth and Hecate intervenes to scramble Macbeth's understanding so that he will fail because of his own pride. Here is part of Hecate's speech from act 3, scene 5:
He shall spurn fate, scorn death, and bearHis hopes 'bove wisdom, grace, and fear.And you all know, securityIs mortals' chiefest enemy.
In Act 3, Scene 1 Macbeth has a long soliloquy beginning:
To be thus is nothing,
But to be safely thus. Our fears in Banquo
Stick deep, and in his royalty of nature
Reigns that which would be feared.
What troubles him the most is that the three witches prophesied that Banquo's descendants would be kings and not his own sons. Macbeth believes that this is predetermined by fate but that he can actually defy fate with his free will by murdering Banquo and his son Fleance. If he can manage to accomplish that, he can cut off Banquo's line completely. He is opposing his free will to fate itself. In the final lines of the soliloquy he says:
They hailed him father to a line of kings.
Upon my head they placed a fruitless crown
And put a barren scepter in my grip,
Thence to be wrenched with an unlineal hand,
No son of mine succeeding. If 't be so,
For Banquo's issue have I filed [defiled] my mind,
For them the gracious Duncan have I murdered,
Put rancors in the vessel of my peace
Only for them, and mine eternal jewel
Given to the common enemy of man [Satan]
To make them kings, the seeds of Banquo kings.
Rather than so, come fate into the list,
And champion me to th' utterance.
He challenges fate to come--metaphorically--into the arena for a trial by combat and fight him [champion him] to the death [to th' utterance]. It turns out that fate wins the trial by combat, because Banquo is killed but Fleance escapes.