Decide which you feel is more responsible for the gruesome events of the play, the witch's supernatural powers, or Macbeth's pre-existing evil unlocked by their simple suggestion. Defend your choice. As part of your answer, it MAY be useful to decide if Shakespeare really wrote all of Act 3, Scene 5.

At first glance, it seems that Macbeth is responsible for the gruesome events of the play. As soon as he receives the prophecies from the witches, for instance, his mind immediately begins to think about killing Duncan. Compare this with Banquo's reaction to the prophecies. When Banquo receives the news that his sons will be kings, he is rather skeptical and actually wonders if he and Macbeth have eaten a certain "root" which has produced hallucinations. The point here is that Banquo does not react with murderous or sinister thoughts, suggesting that he lacks the evil found in Macbeth.
Looking at Act III, Scene V, however, Shakespeare is suggesting that Macbeth has been bewitched by Hecate, who has deliberately misled him and intends to do so again.
On balance, then, it could be argued that Macbeth has been influenced by the witches, but that he already possessed some propensity to evil, making the result even more dramatic and violent.