I would just like to add a little more to the question. Supernatural power introduces evil into the play through the three witches. Their prophecies tempt Macbeth into first killing Duncan. They offer him the opportunity to become powerful, but to gain this power, he must committ this evil deed, which of course leads to other evil acts. The power of the witches lies in their ability to tempt humans by offering them something they really want. Evil can only exist if mankind gives in to these temptations.