Macbeth is a tragic hero because he goes from being thought of as a courageous, heroic, loyal subject of King Duncan to a man who surrenders his soul to the powers of darkness for the sake of possessing power.
Macbeth's downfall is due to unchecked ambition. His desire to be king overwhelms his life, and, once this plan is set in motion, he murders anyone who appears to be a threat to his future as king.
Once Macbeth starts to use murder as a means to solve his problems, he becomes isolated from humanity. He loses his ability to be comforted, he cannot sleep, he is unable to eat, he has hallucinations and is tormented. This leads to his slow descent into madness.
His life held so much promise at the beginning of the play, he was given a second title, Thane of Cawdor, he was looked upon with great favor by King Duncan. By seeking that which he should not have, the throne of Scotland, Macbeth destroys himself, his wife, as well as all the lives of his murder victims.
At the end of the play, he reflects on the futility of life and he grows weary of all of it. He says:
"Life's but a walking shadow, a poor player
That struts and frets his hour upon the stage,
And then is heard no more; it is a tale
Told by an idiot, full of sound and fury,
Signifying nothing." Act V, Scene V
Macbeth is a tragic hero because he evokes both our pity and terror has excessive hubris (pride) and is overwhelmed by his harmartia (tragic flaw). He is a brave military leader and a noble warrior who is good and honest at the play's onset. However because of his 'vaulting ambition,' (tragic flaw), he is willing to corrupt himself by killing his King, his best friend, Banquo, witness his wife's demise, and destroy the kingdom through treachery and mayhem, having many who may stand in his path brutally murdered. He is set on this course of destruction through the witches' prophecy, he hears what his pride wishes him to hear, and his wife's unrelenting ambition.