The Weird Sisters' ability to disappear at will and conjure apparitions to speak to Macbeth about his future convince him of their supernatural powers. But because it is Macduff who delivers the fatal blows to Macbeth, the Weird Sisters are not directly responsible for his death. Their role is in helping to awaken the ruthlessness in Macbeth that accompanies the ambition that he naturally possesses.
Macduff kills Macbeth for at least two reasons: Macbeth has arranged for the murder of Macduff's wife and children, and so Macduff exacts his personal revenge. At the same time, Macduff is a true Scottish patriot, and he knows that it is Malcolm's right to claim the throne of Scotland, not Macbeth.
Macbeth has no fear of Macduff because he interprets the prophecy from the apparition that "no man born of woman shall harm Macbeth" to mean that he is invulnerable. Because these ambiguous words from a supernatural source mislead Macbeth, he takes actions that provoke Macduff's violence.