Shakespeare makes it evident in Act I, sc. i of Macbeth that the witches are evil forces, for they can confuse appearance and reality. Their chanted line "Fair is foul, and foul is fair" resonates this idea throughout the play. They place suggestion in Macbeth's mind that he will become king, and he charges ahead to make it happen.
After the downward spiral of events that occurs after Macbeth's coronation, Macbeth vows to revisit the witches. Before this occurs, the leader of the witches, Hecate, in Act III, announces her intention to lead him "on to his confusion" because "security is mortal's chiefest enemy". She further announces that he will "spurn fate" and "scorn death" because of what he hears.
The witches do use Macbeth as a pawn, but one must be careful to note that Macbeth does retain his sense of free will. The witches do not say how the prophecies will be fulfilled, so Macbeth's, himself, may determine these courses.