How does Shakespeare characterise the witches in Macbeth?

In Macbeth, Shakespeare characterizes
the witches as the harbingers of fate and destiny.  In the first scene of the play, the
witches meet and indirectly state the events that they foresee.  By starting the play
with a scene in which only the witches appear, Shakespeare is suggesting that the play
will be a battle between one man's fate and free will.  This is confirmed in Act 1 Scene
3 when Macbeth and Banquo meet the witches and Macbeth then thinks about how he will
make his prophecy come true while Banquo remains wary and untrusting of the witches. 
Superstition causes viewers and readers to see the witches as evil; however, they words
and actions revolve around "an eye for an eye," not evil (early in the play the witches
talk about getting revenge on a man because his wife treated them
poorly).