Iago is most responsible for the tragic events of the play because he poisons Othello with words. Of course, it is Othello who commits the misdeeds but it is Iago which propels him to do so.
Had Iago not laid so many "traps" for Othello, he would have simply continued to live life as he had. Iago thrusts him into a jealous, murderous rage and is therefore the most responsible for the events of the play.
One could argue that Othello should have kept himself in line and not resorted to violence, but one must sometimes blame the person who lights the dynamite, not the dynamite itself.