The definition of a tragic hero, as stated by Aristotle, is a man of great stature who, as a result of one tragic flaw, falls from greatness. To this degree, Othello fills the requirements. He is held with respect in Venice, where he lives.
"Here comes Brabantio and the valiant Moor."
He is a noble general of the army, and is extremely proud of his position, and the way in which he is regarded in society. He isn't afraid of what others think of him, and is extremely confident in his own abilities and beliefs.
"If you do find me foul in her report,
Othello's fatal flaw is the trust that he places in Iago, his 'ancient', to whom he looks for advice. Iago is angry with Othello for promoting Cassio over him, and he is looking for revenge. He abuses the trust that Othello places in him, and weaves an intricate tale to drive Othello mad. Iago's intent to deceive is signalled early on in the play.
Othello and Iago have been friends for years, and Othello has no reason to doubt Iago's tale of infidelity. This ultimately leads to his downfall, as Iago constructs more and more evidence that suggests (but never outright proves) that Othello's wife, Desdemona, is conducting an affair with Cassio.
"Look to your wife; observe her well with Cassio."
As Othello becomes more and more convinced, he is ultimately talked into killing Desdemona and Cassio. Iago is revealed as a traitor, but it is too late to prevent Othello from suffocating Desdemona. When Othello realises what he has fallen into, he stabs himself and dies.
So, at first appearance, Othello would appear to conform to all the rules regarding what constitutes a tragic hero. However, in my eyes, Othello is not the same kind of tragic her
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