The role of Prophecy in 'Macbeth'

A question i always find myself asking when presented with a story involving a prophecy is, 'Would the results have been the same had they not known about the prophecy?' In all such stories, the characters tend to base all if their actions on the knowledge of the prophecy and these particular actions often lead to the fulfillment of said prophecies. This seems to be the case even when the subject is trying to avoid the fulfillment of the prophecy. The story of Oedipus is a prime example of how prophecies are inevitable.

In regards to Macbeth, I actually find myself conflicted when it comes to the prophecy. Normally i feel sure that prophecies will always be carried out despite anyone's actions; but in this play, the carrying out of the prophecy seemed so dependent on the fact that both Macbeth and Lady Macbeth were aware of it. In the beginning of the play i got the impression that Macbeth was a dutiful and loyal subject to the king who expressed no particular desire to take the kings place. He is approached by the three witches after having had some success on the battlefield and is at that point told of the prophecy.

Upon hearing the prophecy of his future as King, Macbeth is initially stunned and skeptical. His doubt in my opinion is key. I see it as expressing the fact that this is something Macbeth had never aimed for or even imagined for himself. Once Lady Macbeth is made aware of the prophecy it is fulfilled almost immediately. Macbeth kills the king while the true heirs (the princes) decide to flee in fear of their own lives. I believe that had Macbeth not known of this prophecy, he would have simply been grateful for his 'promotion' as opposed to being slightly angered by the prince's promotion. He would have never even made an attempt at the kings life, and the prophecy would never have been fulfilled.