Probably so, if you consider Macbeth's internal conflict where he argues with himself, "If Chance will have me King, then it will happen without my stir." The main difference in killing Duncan and allowing Chance to manipulate events is the wait time. That's where the witches and their prophecies come in...they plant the seed of ambition--not only in Macbeth's heart, but also in Lady Macbeth's heart...where it grows wildly when sunned with devious smiles and nurtured by ruthless determination.
I think if you were to look contextually at both the play's setting and the time it was written, the concept of magic and seerers was somewhat awesome. I would argue that Shakespeare included a mystical element to please the royalty that found it fascinating, however during the 1500's the idea of women as witches with powers was deemed possible, and later in the 1500's and into the 1600's witch hunts began. Whether you believe in witchcraft or not, people during this time found something threatening about these women, therefore I would argue that the prophecy alone could have caused him to take the action he did to get the throne. Yet the witches never explain how, so you can argue that the prophecy may have come true without Macbeth meddling and ultimately killing off everyone. This question is a toughy... you can really defend it both ways.
I think that Macbeth would not have taken the step of assassinating the king in order to take the crown by force had he not been pressured so much by Lady Macbeth. The prediction of the witches gave credence to what must have been Macbeth's wife's ambition well before he met the weird sisters.
That's an excellent question.
Macbeth seems surprised by the prediction of the witches, which suggests that he has not entertained the possibility of becoming king before that encounter. After being crowned thane of Cawdor after the prediction of the witches, Macbeth seems taken by the idea of becoming king. Before his encounter with witches, Macbeth seems to have been a heroic soldier and loyal to the king. Even after his encounter with the witches he seems to doubt whether he should kill the king, and suggests that the only reasons to do so would be ambition. Lady Macbeth, coupled with the prophecy of the witches, seems to lead Macbeth toward killing the king and becoming king himself. Ultimately, it would seem that without the intervention of the witches and also Lady Macbeth, Macbeth would never have taken the steps necessary in order to become king himself.
I have one thing to say in between, ... Only killing Duncan would not have made Macbeth king, he would have to kill Malcolm also... it is sheer luck that Malcolm runs away... is this point important?
The ambition of becoming the king was already there in Macbeth when the witches made their prediction. The witches only exteriorised the presence of 'foul' in 'fair' Macbeth. But for their prophetic proclamations, the virus of evil ambition would have remained in a dormant state, unmanifested because of the immunity provided by his moral scruples & imaginative conscience. Even towards the end of the so-called temptation scene(I.iii), Macbeth decided to leave the whole issue to chance. Presumably, had the witches not predicted about his prospect of kingship, Macbeth would not have written to Lady Macbeth, and she also would not have so volunteered to goad her husband to the murder of Duncan. The witches thus provoked Macbeth to cross over the threshold.
I think there IS a possibility that he would have thought of it eventually on his own, although I do agree that prior to the prophecy, he hadn't really plotted anything out. He was a good man prior do this, but it seems like he was only good because his moral fiber had never really been tested before. In fact it doesn't seem he has any morals at ALL, most of his fears and trepidation are motivated by fear of consequence and not by any actual moral content. He's tormented by terror more than remorse in some ways.
Both he and his wife seem very prone to vivid imaginings, and I think that certain events would have eventually led either one or the other to realise that the crown was indeed within reach. I no doubt think the witches prophecy sped up the action considerably, however they did not bewitch Macbeth with any spell, merely planted the seed of a thought in his mind, wherein his own inner workings allowed it to germinate.
After having honor after honor heaped upon his name, I am quite sure either him or his wife would have eventually seem that the only thing between him and a crown was regicide. So while we can be mostly sure that he did not think of this plan beforehand, I suspect he would have eventually come up with it on his own. Which is why I found the witches meddling to be particularly insidious.