Othello's eventual downfall is caused by his two main flaws: insecurity and gullibility. Both of these traits are portrayed in his irritation to Desdemona in Act 3, Scene 3 (especially in contrast to behavior before). Instead of praising Desdemona and vying to follow her every command, he is rude to her. Desdemona observes this when she says "Why do you speak so faintly? Are you not well?" (Act 3, Scene 3, line 325). This is a result of his trust in Iago. Iago takes advantage of this by claiming Desdemona is unfaithful, fueling Othello's insecurity in his age and race.
it's all coming together...
You are now my lieutenant!
Othello's hubris comes into play when he refuses to believe Desdemona in that she is not having an affair with Cassio. This comes from Iago's manipulation, convincing him Desdemona would never love him because he is older and a black man. This results in him pushing aside anyone else's words, only now being trusting of Iago. (Act 4, Scene 1,
"I found by fortune, and did give my husband"
Othello's fate is sealed once he kills Desdemona in Act 5, Scene 2. This is the ultimate result of Iago's manipulation of Othello's trust and insecurity of his relationship. Iago has him set up, so while he is in the act Emilia comes to speak to him. Othello's murder is revealed when Desdemona calls out in her final moments. She claims "A guiltless death I die" (Act 5, Scene 2, 150) in her final moments.
"Who can control his fate? ’Tis not so now."
Though Othello is never shy to admit his undying trust Iago, it is truly sealed by the end of Act 3 Scene 3. There, he states "I am your own forever" (Act 3, Scene 3, line 546). Othello says this after he and Iago establish a plan to kill Cassio and Desdemona, fueled by the Iago's manipulation. In the play, they are depicted on their knees to depict the unity of marriage, as Iago is now all Othello has.
Othello discovers Iago's true nature when Emilia says it was her who stole the handkerchief. "She give it Cassio? No, alas, I found it, And I did give ’t my husband" (Act 5, Scene 2, lines 274-275). This results in Iago killing his wife and Othello realizing he was duped into believing his wife's affair.
After this revelation, Othello is so overcomes with shame for what he has done and cannot fathom his new image. As a result, he stabs himself, lamenting that he would like to be remembered for the good he had done as well. "Then must you speak Of one that loved not wisely, but too well; Of one not easily jealous, but being wrought, Perplexed in the extreme;" (Act 5, Scene 2, lines 403-406). The play ends with three people dead all due to Othello's own vulnerability caused by his insecurity and trusting nature,