By killing himself, he is cleansing the world of his "inner darkness" being a Turk, the beastliness that ruined the superior and ordered Venetian society. It is this, himself, who he kills -- showing he is, at heart, still a Turk, and not the driving motivation that causes all these events to unfold (Iago) -- as Iago is stabbed but has not died. This signifies the curse of suspicion and reason cannot be eliminated -- reason preys on individual people and is not something one can rid. In the end, he chooses once again to rid the tumor of society (which he believes first is his wife, Desdemona, now it is him, the Turk), following honor rather than personal desire.