Macbeth, in the Aristotelian sense, does possess a fatal flaw. For most who consider this question, it is his ambition, for others it is his want of power. Both can be seen as similar for they both require increasing positions of power from Thane of Glamis, to Thane of Cawdor, to King. His want of power motivates him to kill; his want of power motivates him to listen to his wife - who clearly questions his manhood, which, by extention, questions his sense of power for being a man meant being powerful.
Macbeth's vaulting ambition, though it is what brings him to his height of power, it is also what leads him to his downfall. Vaulting Ambition is Macbeth's only flaw; it disables him to achieve his utmost goals and forces him to face his fate. Without this ambition, though, Macbeth never would have been able to achieve his power as King of Scotland or have been able to carry out his evil deeds. In these instances, ambition helped Macbeth do what he wanted to do. But, consequently, Macbeth's ambition has another face and is what leads him to his tragic downfall. Had he not been so enveloped with becoming King and remaining powerful, he would not have continued to kill innocent people in order to keep his position. It was because of these killings and his overbearing attitude that caused him to be overthrown and killed himself.
Macbeth is too easily led by others into sin. He listens to the witches and their prophecies and begins hoping that something will happen that will allow him to become king. Rather than stop there, he tells his wife, Lady Macbeth, what has been foretold, setting her determination and ambition into motion. And rather than telling her to knock it off, Macbeth allows himself to be pushed into killing King Duncan and claiming the throne for himself.
Prior to the killing of Duncan, Macbeth could have stopped and said, "No, I'm going to do what I know to be right. If I am meant to be king, then it will happen on its own." Instead, he caves into the pressure from his wife (and his own ambition - he's not guiltless in this regard at all) and takes matters into his own hands - a dagger, to be exact!