To develop a thesis statement, first you have to understand what brought about Macbeth's downfall. In Act 1, sc. 7, Macbeth says, in his soliloquy, "I have no spur / To prick the sides of my intent, but only / Vaulting ambition, which o'erleaps itself / And falls on the other-." He says that he has no reason to kill Duncan other than his own ambition which can make a person careless. In Act 3, sc. 5, Hecate, after berating the witches for not doing more to Macbeth, tells them that they can make Macbeth feel overconfident (ll. 30-33) when she concludes with, "And you all know security / Is mortals' chiefest enemy." She knows that if Macbeth feels like he cannot be defeated, he will have his guard down and that's when it is easiest to defeat him. Macbeth lets his ambition control him enough to get him to kill Duncan. After seeing the apparitions in Act 4, he feels like he cannot be defeated by any man born of a woman or until the woods around the castle advance to the castle - both seeming impossibilities. He didn't bother to try to figure out if the witches were tricking him with their prophecies because the prophecies made him feel secure. It is up to you to decide which of these weaknesses brought about Macbeth's fall.