This questions involves two of the most important themes of Macbeth, free will and supernatural intervention (or fate.) Shakespearean scholars disagree on the degree to which the witches are complicit in the murders. It is clear that they mean to manipulate Macbeth to do evil deeds, as Hecate makes clear in her speech in Act III, Scene 5. She says that she:
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.
In many ways, this describes the witches' approach to dealing with Macbeth. They do not actually influence events directly, but rather manipulate Macbeth. In the first meeting, the witches only prophesy the future when they tell him that he will be king. They do not even suggest murder, much less tell him how to proceed. It is up to him to make that happen, or choose to wait for it to happen. Once Macbeth has begun to think about murdering Duncan, it is his wife that pushes him along to carry out what he is resolved to do. But it was ultimately his idea, neither the witches, by piquing his ambition nor his wife by steeling him for the task of murder, made his decision for him.