What influence do the witches' prophecies have on Macbeth's decisions to murder Duncan and to have Banquo and Macduff killed?
One of the key themes and lingering questions about Macbeth involves the role of the witches. Did they cause Macbeth to commit the murders, or did they simply point the way to the future, allowing Macbeth to turn to violence to fulfill his ambitions? In other words, was Macbeth really in charge of his own fate, or was he just a pawn of the witches and their malevolent designs?
However one interprets these questions, there is no doubt that the witches kindle Macbeth's ambitions by hailing him as the future King of Scotland. Prior to this, we have seen that Macbeth's countrymen, especially King Duncan, regard him as a valiant, worthy warrior. Very soon after hearing the prophecy (and finding out that part of the prophecy, that he would soon be thane of Cawdor) has come true, Macbeth is already thinking of its implications, and planning to fulfill his own "black and deep desires." When his wife learns of these developments, she takes her place alongside the witches as a major factor in pushing Macbeth to the murder of Duncan.
As for the murder of Banquo, it is because the witches have predicted that Banquo will "get kings" (i.e. that his descendants will be kings), that Macbeth decides to kill him, as well as his son Fleance, who ultimately escapes. Later, we see that Hecate and the witches are instrumental in pushing Macbeth to murder Macduff, conjuring visions that warn him to beware of him. They also trick him into believing that he is essentially unconquerable, a deceit that results in his death at the end of the play. So the witches are very important in provoking Macbeth, painting an ambiguous picture of his future, and stoking his considerable ambition. His trust in the witches, along with his unchecked ambition, is one of his most glaring flaws.