According to the enotes reference about Shakespearean Tragedies, the protagonist of the play must be an admirable but flawed character.
Macbeth is admirable. He is an honored general in the King's army at the beginning of the play who wreaked vengeance on a traitor. Other characters reference his bravery. In Act 1, Scene 2 Malcolm says,
But Luck and Macdonwald together weren’t strong enough. Brave Macbeth, laughing at Luck, chopped his way through to Macdonwald, who didn’t even have time to say good-bye or shake hands before Macbeth split him open from his navel to his jawbone and stuck his head on our castle walls.
And yet we certainly know that Macbeth was flawed. His flaw lay in his overwhelming ambition. When the witches first predict that Macbeth will become Thane of Cawdor and someday, King, Macbeth outwardly shows no sign of this flaw, but his thoughts of murder begin at this point. In Act 1, Scene 3 he says,
I am thane of Cawdor. If good, why do I yield to that suggestion Whose horrid image doth unfix my hair And make my seated heart knock at my ribs, Against the use of nature?
According to M.H. Abrams, a tragedy is "representations of serious actions which eventuate in a disastrous conclusion for the protagonist". William Shakespeare's Macbeth is his shortest yet darkest tragedy. This play is a tragedy, because:
- The protagonist faces heavy downfall for his tragic flaw: The protagonist or the principal character in the play Macbeth seems to be highly ambitious. Macbeth, who is a heroic figure in the beginning, becomes a butcher at the end, and has to pay huge toll solely because of his devastating ambition.
- The play creates pity and fear in our minds: A brave warrior and patriot being driven by his hamertia brings his own downfall. The readers get scared and feel pity for him placing themselves in his place. The character's downfall is not easily acceptable since he was a "worthy gentleman" (1.2) earlier who "was too full o' th' milk of human kindness" (1.5), and who gets spurred by evil supernatural power along with his wife. The greed inside him arises, undoubtedly, due to the instigation of the evils. And, he gets entrapped by deception which makes us feel more sympathetic towards him. More than that, he shows his courage till his death. His gradual descending evokes pity and fear in the minds of the reader and audience for him to many extents.
This question can be answered more elaborately, but the major points are the above two.