Men. And their reputations. The culture of honor.
You cannot blame Iago solely.
Elizabethan England and Italy were patriarchal cultures of honor, where the male name and reputation were prided above all. As Cassio says:
Reputation, reputation, reputation! O! I have lost my reputation. I have lost the immortal part of myself, and what remains is bestial.
Look at the names men that are given in the play: Duke, Senator, General, Ensign, Lieutenant. And the names women were given at the time: unquiet, wanton, harlot. See a disparity?
Iago is jealous of Cassio and Roderigo because he was passed over for a promotion by Othello, who favored the younger Cassio. Iago's reputation was damaged.
Iago thinks Othello slept with his wife. Not that he cares, but it insults his...Reputation.
Brabantio cannot abide by a man of lower reputation (Othello) having his daughter.
Othello performs an honor killing of Desdemona because he cannot be connected to an unfaithful wife. Reputation.
Emilia steals the handkerchief to get back some form of reputation with her husband.
Desdemona will willingly be murdered so as not disrupt her perfect reputation as the dutiful wife.
Every conflict in the play is incited by an attack on reputation.
In all fairness to Othello, he and Desdemona did not know each other very well in terms of a married man and wife relationship. They had not been married long and had only just enjoyed a few hours or days privacy together - made all the more possible because they were out of sight and earshot of the court. His love for Desdemona is not as strong as her confident love for him. Clever Iago does not waste much time... he sets his mechanics in motion before the loving bond becomes strong enough to withstand doubt - he knows Othello, both in strength and weakness. Every tragic hero bears some responsibility for the tragedy that befalls them - as we all do. We are all only human and are fallible - or as some would refer to it ... we endure 'original sin.' So as far that goes, Othello bears some responsibilty for his own downfall, and of course for the murder as there is no justification for that. Put Othello's weakness and Iago's evil cunning together and tragedy follows. Othello, once confident and well respected with a good reputatation is out of his depth in fancy society when he tries to become part of it by blood. That, they are not going to have - no matter how good and vital his soldiering. Coupled with the low self-esteem he feels in other areas such as color,race,attractiveness and age his wounded pride at his wife's imagined treachery and unwillingness to listen to reason exacerbates the tragedy instigated by Iago. Many critics in England, Shakespeare's birthplace, tend towards this view. It remains however a contentious issue - close reading of the play and the characters link below will help you form a view of your own:
Iago is, of course, mostly responsible for the tragedy because had he not manipulated almost every scene, then the tragic events most likely would not have occurred.
That being said, other characters give in to their weaknesses and allow Iago to rule them; so they are also responsible. Othello has poor judgment especially in whom he trusts and allows his jealousy to rule his actions, something which he states the danger of in Act 2, Scene 3. Desdemona, instead of being honest with her husband about losing the handkerchief lies about it. Both she and Emilia are guilty of not being observant enough so are partially to blame for the tragedy. Finally, Cassio knew his weakness for alcohol and drinks anyway, losing control and allowing the first major step in Iago's plot to go forward. Without his error, it would have difficult for Iago to frame him and Desdemona.
othello is a great liason but it is his actions that cause the tragedy, he should take the main brunt of responsibilty.