Opinion is divided on Iago's ultimate part in Othello's downfall. His role in Othello and indeed his contribution to the main theme of the play cannot be disputed. However, many would claim that Othello should have had the strength of character - at some point - to recognize Iago's motives and question him more convincingly. There is certainly an argument here for the danger of circumstantial evidence in any situation.
Should you feel that Othello is essentially noble and is the pawn in Iago's plan:
the innocent hero falling victim to Iago's schemes and being corrupted by his evil
then you will blame Iago entirely. However, others
argue that Iago's actions merely cause Othello's noble facade to crumble, releasing his inherent savagery.
In this instance, the blame must lie with Othello himself and a serious flaw in his character.
Iago is undoubtedly a master manipulator
honest, honest Iago
and takes advantage of everyone in his efforts to destroy Othello for overlooking him in favor of Cassio and for his own irrational belief that Emilia was unfaithful with Othello:
nothing can or shall content my soul
Till I am evened with him, wife for wife.
Iago and Othello have interestingly been likened to each other in character. This may seem unlikely due to Othello's noble characteristics but both feel
excluded from upper-class Venetian society.
It is perhaps Othello's own feelings of inadequacy that draw him in and make Iago all the more believable as he too shares this same sense of inadequacy.
There is a discrepancy in Othello between appearance and reality
men should be what they seem
ironically uttered by Iago when he is the very opposite of this by his own admittance:
I am not what I am
Iago's deception is what drives the characters in Othello thereby compounding the belief of his responsibility for the outcome of this tragedy.
Iago contradicts his own supposed beliefs in the importance of a man's reputation when he talks to Cassio
Reputation is an idle and most false imposition
and later of Othello, leaving the audience to draw their own conclusions as to his actual beliefs
makes me poor indeed
Ultimately then the decision to blame Iago for the occurences and outcome lie in the interpretation. What cannot be overlooked however is that a need to be accepted and favored can sometimes create the devil inside.