Macbeth’s downfall is his increasingly brutal murders (of Duncan’s servants, Banquo, Lady Macduff and her son). Macbeth’s second meeting with the witches foresee's Macbeth’s downfall when he has his final confrontation with Macduff and the opposing armies.
"Something wicked this way comes" (page 41 line 45) This means Macbeth has a huge fear of the witches, as they are the harbrigners of prophecies, but they can also be the bringers of death, as as soon as they told him the prophecies, he killed all those who were close to him.
'The Three Witches Appearing to Macbeth & Banquo' - - - Henry Fuseli, R.A. (Zurich 1741-1825 Putney Hill). Oil on canvas. 34 3/8 x 44" - -- -'Fair is foul, and foul is fair.' - The Three Witches, Macbeth.