I think that is Macbeth had not heard the response eventually he would have still become king just not in such a bad way, I think the prophecy was always going to be true but because the witches are evil they almost tricked him into rushing into the kingship because, they wanted something bad to happen and they knew that this would be the perfect time, he went to the king and killed him because the witches told him he would become king and he rushed into it by killing the previous king.