Macbeth is Shakespeare's most compelling psychological drama, and therefore, much has been written about the play. Some people feel the witches cause Macbeth's downfall, and some think Lady Macbeth encouraged the events which caused his demise, but truthfully? I think it was Macbeth, and his ambition.
Because he was overly and pridefully ambitious, he latched onto the witches and his wife to support his own deeds. For instance, when he meets the witches in Act I, scene 3, they already know he is coming, and they say "Peace, the charm's wound up" meaning what he hears will motivate him to react.
Check each of the other scenes you have listed for proof, using this link http://www.enotes.com/macbeth-text , which has both Shakespeare's text, and the modern translation to help you.
Good luck! Remember, whichever character you choose, you must support your choice with the literature.