How are the witches powers limited in Macbeth?

The witches represent the evil in men. Their prophecies just activate what is already present in men's souls and hearts. For example, the witches tell Macbeth he will become king. This becomes true since Macbeth wants to believe in this prophesy and commits any crime to fullfil his dream. The power of the witches are limited because the nature of the human being make things happen, not the witches. Human nature puts limits on the witches' power.