Macbeth's transition of Power

As we all know, the witches prophesied that Macbeth would become king of Scotland.  Lady Macbeth and Macbeth came to the conclusion that He needed to kill Duncan.  Was there any other way that he could have become king?  Kings very rarely cede their power to another man.  It would take a sign from God himself to put Macbeth into power.  Even if the witches were public about their prophecy, much controversy would arise as to the legitimacy of hags who are not priests.  If Duncan were to die, his sons would be the immediate heirs.  Then his two sons would have to die before they had kids, and Macbeth would have to be the greatest thane of Scotland to take the throne.  He was already on the right track after, being thane of two lands already.  What the answer to this question comes down to is belief.  Is prophecy so concrete that it must come true, or does it require action?  Is prophecy a telling of the future or an immediate license to the future's ends?  I feel as though Macbeth's desire for power would have made him blind to the thought of any other outcome, but if prophecy is always true, his killing of Duncan would not have been necessary.