The witches in William Shakespeare's Macbeth have been described as numerous things. They were called "instruments of darkness" and "fantastical" by Banquo and "imperfect speakers" and "black hags" by Macbeth. Some readers believe that the witches are truly evil (based upon stereotypes of witches); and others may believe them to be the voices of the Fates (as seen in Greek mythology--ones who would determine a man's destiny). That said, the witches only provided Macbeth with a glimpse into his future; it was his hand which forced the removal of Duncan from the throne.
Therefore, depending upon one's individual interpretation, a reader could support his or her own defining of the witches using different textual support. If following Banquo's point of view, one could deem the witches as evil. If following Macbeth's train of thought, one could deem the witches as evil as well. If looking at the allusion made ot mythology, one could easily state that the witches represent, or symbolize, the Fates. Lastly, one could suggest that the witches symbolize opportunity and the extent one may go to if another believes the opportunity exists.