Macbeth is a tragic hero in that he's a fundamentally decent, noble man brought low by a character flaw. At the start of the play, Macbeth has just acquitted himself bravely on the field of battle. More than anyone else, he's responsible for the crushing victory over the Norwegians. King Duncan is so grateful for Macbeth's incredible valor that he rewards him with the title of Thane of Cawdor. Yet somehow this just isn't enough. Macbeth's head has been turned by the Weird Sisters' prophecy, his own ambition, and the machinations of his wife. He realizes that the throne of Scotland is within his grasp if only he can make that final, fateful step and murder Duncan.
But still he has to be persuaded of the rightness of this course of action. This is where the tragic dimension of Macbeth's character comes into play. Treachery and murder are not really a natural part of his personality. He's not an inherently evil man like Richard III, who positively revels in his diabolical wickedness. Macbeth still has a conscience, which is why he always appears so uneasy with the decision to murder Duncan and why he has to be cajoled and manipulated by Lady Macbeth into carrying out the dirty deed.
Even once he's established himself on the throne, Macbeth doesn't exactly luxuriate in his newly won power. He has come to see himself as almost the plaything of supernatural forces. He's a bit player in a gigantic cosmic drama in which the forces of good and evil are engaged in a titanic struggle for supremacy. Although Macbeth becomes more and more like a tyrant, violently lashing out at anyone he perceives as a threat to his rule, we sense that this isn't the real Macbeth, the brave and noble warrior who earned the undying admiration and gratitude of his former king. Instead, this is a man whose very soul has been corrupted by the forces of darkness, whose fatal flaw of ambition has been twisted to serve some very dark and diabolical ends. More than anything else, it is this complete transformation of character, from noble, valiant warlord to cruel and vicious tyrant, that makes Macbeth a tragic hero.