Othello is easily deceived by Iago. He trusts Iago more than he trusts his wife. Othello is deceived by Iago because Iago is a master at manipulation:
Iago weaves an intricate plot to undo the Moor [Othello]. What drives Iago throughout the play is a manipulative duplicity which is inherent in his nature.
No doubt, Iago is a clever man. He is a master at deception. In Act II, Scene III, Othello expresses his trust in Iago, insisting that Iago is an honest man:
Iago is most honest.
Truly, Iago is brilliantly deceptive. He convinces Othello that he loves him:
My lord, you know I love you.
Othello believes Iago. He has no reason to suspect that Iago is lying. Othello claims Iago is an honest man:
...I know you are full of love and honesty
And weigh your words carefully before saying them,
Othello has much admiration for Iago. He believes Iago holds back when it comes to criticizing others. Clearly, Othello is too trusting. He believes that Iago is his honest ensign. Othello could never imagine that anyone who works for him could be deceitful. Othello believes Iago because Iago is very convincing. He is an expert actor.
Truly, Iago plants seeds of doubt in Othello's heart and thinking. Iago tells Othello that Desdemona was deceitful when she married Othello. Desdemona married Othello secretly, thus keeping her father from knowing the truth. Iago is very smart when he turns Desdemona's actions against her. Iago shares his feelings on Desdemona's deceitfulness:
She deceived her father by marrying you;
And when she seemed to shake and fear your looks,
She loved them most.
This statement is a very intelligent accusation on Iago's part. Othello begins to think about his wife's secretiveness. Othello begins to analyze the fact that Desdemona was not honest with her father about her marriage to Othello. Here, Iago makes a good point. If Desdemona would lie about one thing, she may lie about another. Truly, Othello is beginning to distrust his beautiful wife. Iago's scheme is working.
Iago continues to manipulate Othello by claiming that he truly loves Othello. Of course, Iago insinuates that he only mentioned that Desdemona had been untruthful with her father because of his love and concern for his leader Othello. Iago pretends to be worried that his words against Desdemona have upset Othello:
I see this has dashed your spirits a little.
Othello assures him that he is not upset, but Iago continues to express his pretentious fear that he has upset Othello. For this reason, Iago once again professes his love for Othello:
Trust me, I fear it has.
I hope you will consider that what I have spoken
Comes from my love;
Again, Othello expresses his complete trust in Iago. Othello also believes Iago is an extremely educated man when is comes to interpreting the actions of others. No doubt, Othello believes Iago is a very honest man:
This fellow's of exceeding honesty,
And knows all qualities, with a learned spirit,
Of human dealings.
Ultimately, Iago's plot is fulfilled when Othello murders his beautiful wife. Othello does not learn that Iago is an evil villain until Desdemona is dead. When Othello learns the truth, he kills himself.