Although Iago's evil machinations lead Othello to his downfall in the play, it would not be correct to say that he is 'only' a victim of these events.
Othello is presented with opportunities to hear from other characters on the suspected affair between Cassio and Desdemona. In Act IV, Othello asks Emilia, Desdemona's maidservant and wife of his 'honest' Iago, about the possibility of a liaison between her lady and Cassio-
I durst, my lord, to wager she is honest,
Lay down my soul at stake. If you think other,
Remove your thought; it doth abuse your bosom.
She swears on her life that the allegations are untrue, and in fact she will die defending her cause. There is a warning in her words also about the source of the information he has received-
If any wretch have put this in your head,
Let heaven requite it with the serpent's curse!
Othello chooses to ignore her words and continue in his path of destruction.
Equally, Desdemona herself professes her innocence, and Othello again chooses to dismiss her pleas.
At the beginning of the play Othello called on Desdemona to testify before the Duke how their relationship began. However, by Act IV he is unable to listen to any voice other then the poisonous Iago. Othello proves to be a poor judge of character, however valiant he may be in battle. His own misplaced trust was his destruction-
one that loved not wisely but too well;