Aristotle describes a tragic hero as one who goes from good to bad, not one who goes from bad to good. He also states that the tragic hero is not initially eminently or completely good. The tragic hero is recognizably good either in reputation or deeds. The tragic flaw (hamartia) is some frailty that leads the hero to make a bad decision.
Aristotle also suggests that the audience pities the tragic hero. It's really up to the reader to determine whether or not Macbeth deserves our pity. If we look at him as an impressionable man, persuaded by the witches' prophecies, thinking they were supernatural and therefore accurate predictions of his fate, then his pitiable flaw is impressionability, surely one we can sympathize with.
But if his flaw is pride or thirst for power, it is more complicated. But like Aristotle's description, the tragic hero has to be good and then become bad. Macbeth begins the play as a noblemen and a successful soldier, loyal to the king. Also, consider that Macbeth is plagued by guilt. This suggests traces of his former virtuous self. If we can't be sympathetic with Macbeth after multiple killings, we may pity him because of his fall from virtue to evil, a fall which may have as much to do with supernatural influence and subsequent psychological turmoil as it did his own pride.