That is one of the major questions in Macbeth. Do the witches simply predict the future, or are they influencing Macbeth in some way? Are they telling him the future with malevolent purposes or in order to test him? This issue raises all sorts of questions about fate and free will. Do Macbeth’s conscious choices determine his fate, is the manipulative Lady Macbeth the truly evil influence here, or is Macbeth inexorably drawn to this dark path?
It is interesting to compare Macbeth to others. Banquo hears the witches’ prophecy, but he does not take the actions that Macbeth takes. On the one hand, he does not have the time to reveal his dark intentions, if he has any; Macbeth kills him too soon. On the other hand, Banquo seems cautious from the beginning, stating that “oftentimes, to win us to our harm, / The instruments of darkness tell us truths.”
Another character to look at is the former thane of Cawdor. Is it a coincidence that two thanes of Cawdor in a row betray the king? The previous thane was a “prosperous gentleman” and a trusted subject of King Duncan. One wonders if he also stumbled across the witches.
The witches come across as sinister creatures, using a variety of unsavory ingredients in their brew, who seem to be ill-intended. However, it is not clear whether they draw their victims towards wickedness or if they simply reveal what is already there.