It might be a tragedy of fate after all. Yes, I do think that the witches play a central role in the play, because if it hadn't been for them, none of the tragedy would have happened. If they hadn't told Macbeth about what was to happen, he would never have thought about killing Duncan and becoming king himself. He would have been satisfied with where he was at that particular moment, and proud of himself. This way, the witches destroyed everything by planting the seed of greed in Macbeth, and thus it had to end tragically.