The witches seem to really enjoy messing with Macbeth. Hecate scolds the three weird sisters later for casting spells and influencing Macbeth. They were apparently not supposed to be doing that. They didn’t have the authority.
Good question. We do not know the motivations of the witches in Macbeth. By openly prophesying that Macbeth will someday be king, the witches are almost certainly intentionally sparking a disastrous desire for power in Macbeth and Lady Macbeth. However, we do not know why.
We do know that the witches can be vengeful. Early in the play one of them tells a story in which she recounts how she harassed the husband of a woman who offended her. Perhaps the witches were getting even with Macbeth for some reason.
(The entire section contains 4 answers and 389 words.)