In the play Macbeth by william shakespeare why were the witches NOT to blame for macbeths downfall?

Excellent question - do the witches make Macbeth do anything? There's no evidence for it in the play. The witches tell Macbeth a future that may or may not be true, but they don't make him do anything. It's Macbeth's nature that he either tries to make this "prophecy" come true, or else the "prophecy" was always going to come true and the witches were just telling the truth. Therefore, they either told him his determined future, or they told him something which, knowing his character, would push him into action. If you know someone who is jealous but happily in a relationship, you could really do some damage by planting the seed in their mind that their partner might be cheating. That's the plot of Othello. Ultimately, though, it is the main characters' only weaknesses which they either overcome or they don't. That they don't isn't the fault of the witches. The witches are putting a bottle of vodka into the hands of a potential alcoholic - it's the alcoholic's choice whether to drink.