Actually, I'm no longer that sure about this. Does anyone have a source for prophecies being unavoidable? They certainly seem to have a knack of getting themselves fulfilled (see October 31st) but are they literally unbreakable?

If Quirrell thinks that he can prevent a prophecy from coming true, he's committed this time around to do it properly and not to try to creatively fulfil it. He doesn't appear to think this is futile.