Othello's tragedy is most attributed to jealousy and a lack of self-worth as a husband. It is because Othello is so insecure in his personal relationship with his wife Desdemona that Iago is able to manipulate him so effectively.
At first Othello is confident in his marriage to Desdemona, but little by little Iago convinces him that she cannot be trusted. He reminds Othello that Desdemona already deceived a man she loved dearly, her father. Othello becomes confused, suspicious and eventually consumed with jealous rage at the thought that his beautiful wife is cheating on him. This thought so repulses the honorable and dignified Othello, that he responds as if on the battlefield.
"Yet she must die, else she'll betray more men.
Put out the light, and then put out the light:
If I quench thee, thou flaming minister,
I can again thy former light restore,"
Othello retreats from being a loving husband and concentrates on being a soldier. He determines that Desdemona is a menace to his life, and he must get rid of her. Even though he loves her, he feels compelled to kill her out of a sense of honor.
"She turn'd to folly, and she was a whore"; "She was false as water"; "Cassio did top her" (V.ii.133; 135; 137). Desdemona, Othello believes, has betrayed him and the sanctity of marriage, and she paid with her life."
Shakespeare tends to deal with contrasting pairs: so in the way that Romeo and Juliet is about both love and hate, Othello is about jealousy and its opposite, faith (trust!).
Othello actually pledges his life on Desdemona's faith early in the play, which comes full circle: of course, Othello should have trusted Desdemona, for she didn't sleep with Cassio. Othello trusts her at the start - and Iago gradually erodes that trust, planting seeds of suspicion and staging false "evidence" which persuades Othello that he has been cuckolded, and so, must murder his wife. Othello - fatally - trusts Iago.
And Iago, crucially, is the one who makes that change happen. He is the architect of the tragedy: yet how far he creates the tragedy and how far he plays on insecurities already present in Othello's mind depends on your reading of the play. It's notable though that Iago plays on Othello's otherness - Iago tells him he knows about the Venetian women, as he is a Venetian - to justify Desdemona's supposed infidelity. And, when Othello lists the reasons, he famously includes "haply for I am black".
Why does Iago do it though? Well, even Iago himself denies us the answer: "Demand me nothing", he says, in his final lines of the play, "what you know you know. From this time forth, I never shall speak word". That silence refuses the characters - and the audience - the justice of a motive.
The tragedy of Othello is not about trust, or lack of trust. It is anger. All the main characters display anger management problems and because of this they become their own victims of tragedy.
Iago says, "Beware the green-eyed God of Anger my lord" when trying to trick Othello into becoming angry!!! Because Iago is angry too. He makes Micheal Cassio angry by giving him water to drink instead of wine. He makes Desdamona angry by stealing her handkerchief.
Iago spreads anger like fire among all the main characters and they fall into angry tragedy because they can't control themselves.
In the end, Desdamona murders Othello because she is so angry with him.