One of the central issues that you need to think through when approaching this play is whether Macbeth was destined to do what he did or whether he had a choice. This is a central question in many works of literature, and how we respond will shape our whole view of this work. In particular, how you decide to answer this question will greatly impact the way you view the role of the witches: are they just reporters of what is going to happen, or do they deliberately manipulate and obfuscate Macbeth, tantalising him with possible futures to get him to act against his better conscience?
Hecate appears for the first time in Act III Scene 5 of the play, where she berates the other witches for not consulting with her about their dealings with Macbeth. It appears that Hecate regards their work with Macbeth as "riddles, and affairs of death," which would suggest that what they have said to Macbeth is not fated. However, later, Hecate remarks that Macbeth will come tomorrow "to know his destiny," which perhaps suggests that the witches are able to tell him what is fated. However, the end of her speech definitely suggests that the witches and Hecate deal in spells and illusions and not reality or destiny:
And that, distill'd by magic sleights,
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...
Therefore, as a result of the "artificial sprites" that Hecate will produce, Macbeth will be led on in his "confusion," leading him to "spurn fate." These comments therefore suggest that Macbeth is not fated to do what he does, he has a free will that the witches seek to manipulate.