Who is responsible for the tragedy in Othello?

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.