Let us be very careful here. Your question seems to suggest that there is a direct causal link between the witches and the crime of regicide commited by Macbeth and his wife, and that the witches "caused" it to happen, as if they had the ability to control Macbeth and his wife like robots. Their influence is far more subtle than this, and let us remember that Banquo is given a prophecy that guarantees his heir the throne, yet he does not seek to commit regicide and kill his king.
Thus the art of the witches lies in their ability to present Macbeth with a vision of a possible future that haunts him. Note how he responds to the partial fulfilment of the prophecy in Act I scene 3 when he is given the title of Thane of Cawdor:
This supernatural soliciting
Cannot be ill; cannot be good: -
If ill, why hath it given me earnest of success,
Commencing in a truth? I am Thane of Cawdor:
If good, why do I yield to that suggestion
Whose horrid image doth unfix my hair,
And make my seated heart knock at my ribs,
Against the use of nature?
Macbeth himself here recognises the ambiguity of the prophecies he has received. The subtle influence of the witches therefore is based on the way that they do nothing more except present their prophecies, full of half truths, and then sit back and watch how they impact the characters. The way that Macbeth and his wife act on his prophecies to ensure their fulfilment points towards some inner character flaw or evil that Banquo, in the way that he does nothing in response to his prophesy, is obviously not subject to.