Macbeth is one of Shakespeare's tragic heroes in that his character follows the model of Shakespeare's other tragic heroes. He is an admirable, powerful, and well respected member of his society, a society in which he occupies a high position. (This can also be said of King Lear, Brutus, and Hamlet, for example.) Like these other tragic heroes, Macbeth's character is flawed, and it is this character flaw that leads to his destruction. Macbeth, like Shakespeare's other tragic heroes, is ultimately responsible for his own destruction.
The nature of Macbeth's fall is epic. From his former position of great respect and admiration, he descends--step by step--until he becomes even more than a villain; he becomes a monster. I've often wondered why Shakespeare makes Macbeth so thoroughly detestable among his tragic heroes. I think the answer lies in Macbeth's crime (regicide) and the English political structure of Shakespeare's time (monarchy). The play develops a very strong statement about killing one's king out of political ambition.