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.
Witches aren't creatures who seek to do good, normally. They represent an evil force, an up-to-know-good persona derived from darkness and all that is bad. And they didn't have to do very much to spark Macbeth's ambition. He could have regarded them as quacks, but he bought their story and cowered under his wife's belittling. I agree with rrteacher that they simply unleased a monster; little did Macbeth know that part of his monster was attached to his wife. I don't think he would have gone through with killing Duncan had his wife not been a witch herself. Hence, all witches in the play were selfish and used Macbeth to fulfill their own goals in the story.
There is no question that the witches are a malignant force in Macbeth, but whether they are in control of Macbeth's destiny, or can simply see what is going to happen, is open to debate. Hecate seems to suggest that she is meddling with his future when she tells the witches that she will conjure up the visions that will confuse him and give him a false sense of security, but again, strictly speaking, she is only telling Macbeth what is going to happen. The interplay between supernatural forces and Macbeth's considerable ambition is one of the central, and most compelling, themes in the play. Would Macbeth have pursued the murderous course of action that he did without meeting the witches? It is difficult to know, but the witches know what they are doing when they reveal their prophecy to Macbeth--they are unleashing a monster.