Question

the witches are sometimes seen as representing fate, or destiny. Do they merely reveal what will happen, or do they manipulate events?

Answer 1

In my opinion, they do not only reveal what will happen, but also manipulate events. When Macbeth approached the witches, they could have concealed the truth from him and just let the events unfold, but they didn't do it. They told him about the prophecy, thus initiating the events that soon followed. If they hadn't told him the truth, Macbeth might have never killed Banquo, Duncan, and the rest in order to become king. So yes, they manipulated events and caused a great tragedy. 

Answer 2

In Macbeth (1623) by William Shakespeare, the witches are seen as representing destiny but they only manipulate events.

They do not predict the future, they only make assumptions based on their knowledge about Macbeth's love for power. Furthermore, the witches, who have great power in the play, use their ability to persuade in order to convince Macbeth and Banquo that the prophecies are real. Therefore, the witches end up being also responsible for Macbeth's actions and crimes. In the play, Macbeth does not control his own destiny, the witches do. Macbeth kills Banquo because they "predict" that his friend will be the father of a line of kings. In that way, the witches not only manipulate Macbeth's fate but they also represent his darkest side.