Those witches are b*tches

Those witches are insane!! They went to torture a ladies husband just because his wife wouldn't share her chestnuts with them. Like go buy your own witch!!! You probably have warts and no one wants to get your nasty diseases! 

I personally don't think the witches are psychic,  I think they are just saying a possible outcome if Macbeth chooses a certain path. And because Macbeth and banquo hear of there possible amazing futures of course there gonna wanna accomplish this "prophecy" . 

Those witches can't be trusted though in my opinion.