Othello acts rashly and hastily—is he as much to blame for the tragedy as Iago is?

Othello's tragic downfall is in his gullibility and lack of good judgement. While he is easily swayed in his opinions about others, his opinions are seemingly fixed unless an outside voice leads him to question his own beliefs. In this, Othello never questions the "goodness" of Iago--nobody calls Iago's character into question until the end of the play. Therefore, Othello never sees "cause" to question Iago's character.

Iago's entire plot hinges on the knowledge that...

Othello's tragic downfall is in his gullibility and lack of good judgement. While he is easily swayed in his opinions about others, his opinions are seemingly fixed unless an outside voice leads him to question his own beliefs. In this, Othello never questions the "goodness" of Iago--nobody calls Iago's character into question until the end of the play. Therefore, Othello never sees "cause" to question Iago's character.

Iago's entire plot hinges on the knowledge that Othello is gullible. We see this in Act 1, scene 3 when, plotting to slander Cassio and Desdemona, Iago says of Othello,

"The Moor is of a free and open nature,
That thinks men honest that but seem to be so,
And will as tenderly be led by the nose
As asses are."

Without this character trait, Iago's plot would never have worked on Othello. And while the relationship between Desdemona and Othello seems solid and loving at the beginning of the play, it is very easy for Iago to convince Othello of her unfaithfulness. In fact, though Othello asks for proof beyond a shadow of a doubt in the matter ("Make me to see't; or, at the least, so prove it, That the probation bear no hinge nor loop To hang a doubt on; or woe upon thy life!" 3.3) he receives what can only be characterized as insubstantial evidence of Desdemona's unfaithfulness. Iago claims to have found the handkerchief in Cassio's chamber, and Othello only sees Cassio engaged in conversation that he believes to be about Desdemona--he doesn't actually hear any of the conversation. Again, he trusts Iago completely.

Furthermore, Othello never directly speaks to Cassio or Desdemona on the matter. He uses the information he receives from Iago to make all of his decisions. And while Iago is masterfully manipulative, it is clear that were Othello not so easily swayed, the tragic outcome of this play might have been avoided.