In Othello, the audience is first introduced to Iago who is a manipulative character, admitting that "I am not what I am"(I.i.66) and making it clear to Roderigo that he will "serve my turn"(42) upon the unsuspecting Othello. Othello believes that Iago is his most trustworthy and "honest" friend and therefore never doubts his integrity. As a brave and decorated soldier, Othello mistakenly believes in the valor and honor of any other soldier so it makes more sense to him to trust Iago rather than his beloved Desdemona.
Othello's fatal flaw, essentially his jealousy, then stems from this mistaken faith in his profession over his emotions. Othello already feels somewhat compromised and perhaps undeserving of Desdemona because he is a Moor. He has had to prove himself loyal to the Venetians and in so doing has captured the heart of Desdemona. Although he proudly maintains that "she loved me for the dangers I had passed"(I.iii.167) and that "she is with me"(268), he still accepts the "ocular proof"(III.iii.364) supplied by Iago rather than the absolute denial of Desdemona. He appreciates her beauty but cannot perceive that the "filthy deeds" (V.ii.151) that Iago has persuaded Othello that she committed are, in fact his own design and, "honest, honest Iago" (157) is the villain.
To ensure that he is remembered as a trusted soldier, Othello, the "honorable murderer"(297) reminds his audience of his service to the State and asks forgiveness based on the fact that he "lov'd not wisely but too well"(347), accepting that his jealousy, not often roused, knows no bounds once he is provoked or "wrought"(348) in the extreme. He does what he believes honorable- kills himself- thereby ensuring his position as a tragic hero.