Othello is a well-respected general in the Venetian army such that the Duke of Venice summons him to discuss tactics against the Turks early in Othello. Almost simultaneously, Iago is already plotting to destroy Othello and has raised Desdemona's father from his sleep, lying to him about Desdemona, having been "transported....to the gross clasps of a lascivious Moor" (I.i.125-127)
It is therefore confusing but deliberate that Othello should be called before the Duke both in his capacity as a general and on a personal level as Brabantio claims- based on "honest, honest" Iago's words - that Desdemona has been "corrupted By spells, and medicines"(I.iii.60) in marrying Othello.
The Duke likes and respects Othello and, once he is satisfied that Desdemona "may profess Due to the Moor" (188), he is anxious that Othello must leave to fight in "this more stubborn and boisterous expedition."(228)Brabantio, although satisfied with Desdemona's account does provide an opportunity for the audience to recognize the beginnings of the seeds of doubt being sewed in Othello's mind: "She has deceiv'd her father, and may thee." (293)
Desdemona is allowed to follow Othello to Cyprus, ironically escorted by Iago whom Othello describes as "a man ...of honesty and trust."(284)
Iago, although passed up by Othello in favor of Cassio is , in Othello's eyes a good soldier, implicitly trusted. Iago cannot understand why he was overlooked when Othello "of whom his eyes had seen the proof" (I.i.28) knows how good a soldier he is. Othello is an honorable man and would not expect Iago to question his motives. Soldiers go where they are needed and so Othello has no idea how upset Iago is that he "rather would have been his hangman."(34)
Othello kills Desdemona to "thy former light restore" (V.ii.9) and despite her protestations maintains that she gave her handkerchief to Cassio. It still does not occur to him that Iago may have lied. The "ocular proof" - the handkerchief is more than enough proof for him. It is Othello's belief that Desdemona's death is "honorable...for nought I did in hate, but all in honor" (298)
Othello so trusts the institution of the military and therefore Iago to such an extent that this sets him up to believe Iago, a soldier. over his loving and faithful wife. Ultimately this destroys the relationship between Othello and Desdemona as he only sees too late that Iago, his trusted ancient and supposedly an honorable soldier is nothing more than a "devil."
Othello kills himself to preserve the belief that the "malignant and a Turban'd Turk "(V.ii.356) has "done the state some service." He cannot be excused for killing Desdemona but should also be remembered for being a good soldier. This confirms his misguided belief in what he thought Iago stood for.