In response to this question, the reader must keep in mind that Elizabethans felt that the supernatural world was in direct competition with the natural world. Ghosts, especially, were thought to have a profound effect upon the natural order of events. Taking advantage of these Elizabethan beliefs, Shakespeare employed elements of the supernatural world in order to create dramatic emphasis rather than direct affect upon the actions of characters.
That Macbeth wishes to make use of the supernatural as cause for his actions is apparent in the first act:
If chance will have me King, why, chance may crown me,
Without my stir. (1.3.155-156)
But, he does not deceive himself long about the influence of the supernatural being all that effects events. For, in his ambivalence regarding murdering Duncan, he admits to his tragic flaw:
...I have no spur
To prick the sides of my intent, but only
Vaulting ambition, which o'erleaps itself
And falls on th'other--(1.7.25-28)
In subsequent acts, also, Macbeth does not deceive himself. In Act II, for instance, he tells Lady Macbeth,
I'll go no more
I am afraid to think what I have done. (2.2.50-51)
And, in this same scene, Macbeth regrets his actions without any blame attributed to fate:
Had I but died an hour before this chance,
I had lived a blessed time; for from this instant
There's nothing serious in mortality. (2.3.99-101)
As his paranoia increases, Macbeth does consult the witches, but his actions in response to their predictions are more in defiance of the supernatural rather than in accord with it, thus providing the dramatic effect mentioned earlier. In Act V, Macbeth acknowledges the evil he has committed:
...My way of life
Is fall'n into the sear, the yellow leaf,
And that which should accompany old age,
As honor, love, obedience, troops of friends,
I must not look to have, but, in their stead,
Curses not loud but deep, mouth-honor, breath,
Which the poor heart would fain deny, and dare not. (5.3.25-31)
Thus, in his "vaulting ambition which o'er leaps itself," Macbeth chooses to allow the predictions of the three witches to motive his own actions to be king.
I agree that a case can be made for either position; however, the stronger position is clearly Macbeth being responsible for his own actions. Let's just say the witches predictions/forecasts were all true. If that's so, and the first one came to pass without any action on his part (being named the Thane of Cowdor), he should have assumed the same would be true for the rest. Instead, he goes off on a quick tangent (with the help of his wife, of course) and plans to kill the King. Even there he had more than one opportunity to let fate control matters--especially when his wife came back from Duncan's room and said she just couldn't do it. Clearly Macbeth didn't believe this would happen just by fate (as represented by the witches); he felt he had to do something on his own to make it all happen. He does, of course, and he causes his own ruination and eventual demise. He's in control of his own actions.
You might actually come down either way on this, I am sure you could argue both sides, but I would contend that Macbeth is ultimately responsible. The witches don't actually have a great deal of control over the lives of the people they prophesy about, but they have enough information that people trust what they say as the truth.
The problem with this arises because Macbeth trusts what they say without thinking that they might not be giving him the whole story. In this manner, they use their power over information to guide his decisions and push him in certain directions. But Macbeth is the one that makes the choices to follow their advice, often against his own better judgment and his intuition.
A shorter answer: Macbeth is fully responsible for his own actions, the Witches and their prophesies are but mere reflections of Macbeth's thoughts, yes, the witches are giving him a push towards the deed by bringing up the posibility of it, but the deed is not done until it is done and it is Macbeth's choice to act or not, this is what makes Macbeth one of the most human characters in Shakespeare.