Macbeth believes that the witches are supernatural beings because he assumes that "they have more in them than mortal knowledge." He regards them as the agents of fate as soon as their first prediction turns out to be true. When the title of Cawdor is transferred to Macbeth, he is convinced that the rest of their prophecy will come true -- that he is to become the king:
Two truths are told,
As happy prologues to the swelling act
Of the imperial theme.
Macbeth is blinded by his own ambition to become the king, so he takes what the witches say very seriously and decides to commit himself to make their prophecy possible. He constantly depends on the witches and has succumbed to the power of their prophecy:
I burned in desire
to question them further...
He fails to see them as the masters of manipulation and naively believes in anything they say. However, they should not be the ones to blame for his downfall because Macbeth himself committed all the horrible deeds. The witches did not make him do anything.