ash; it strokes his already large ambitious nature to hear that he is going to become King. Although I don't think Macbeth would have done the things he does unless the Witches had met him, it does seem to be that the desire for things like kingship were already in Macbeth; it was the final straw to meet the Witches and their visions of him as King finally tip him over the edge into actually trying to attain the title.
             However, it doesn't seem to be ONLY the Witches' influence that makes Macbeth do the deeds he does. For example, just because Banquo's children are predicted to become Kings ("Thou shalt get Kings, though thou be none," in A. 1, S. 3, Ls. 68/69) Banquo just doesn't go and murder the King! Banquo is a little more suspicious of the Witches:
             "To win us our harm, the instruments of darkness tell us truth." (A. 1, S. 3, Ls. 123/124)
             This stops him from truly trying to fill out the prophecies. Perhaps because Banquo is nobler, less trusting and less ambitious than the Macbeths
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