Iago uses insinuation and treachery to play on Othello's fears that Desdemona does not truly love him. Through lies and manipulations, he convinces Othello that Desdemona is having an affair with Cassio. Othello feels honor bound to kill Desdemona. He does so, only to have Emilia reveal Iago's hand in having had her steal Desdemona's handkerchief so that he can make it look as if Desdemona gave it to Cassio.
When Othello realizes in act 5, scene 2 that he has made a terrible mistake in killing Desdemona, he rises to heroic grandeur by taking full responsibility for what he has done. Before he commits suicide, he remarks, going over his life with the melancholy of one soon to die, that he has done the state some good services as a soldier, then tells his listeners that they must tell his story honestly, neither exaggerating his value nor maliciously making him look worse than he is. He says:
I pray you, in your letters,
When you shall these unlucky deeds relate,
Speak of me as I am; nothing extenuate,
Nor set down aught in malice.
He confesses fully to his crime of killing his wife, saying:
Then must you speak
Of one that lov’d not wisely but too well;
Of one not easily jealous, but being wrought,
Perplexed in the extreme; of one whose hand
(Like the base Indian) threw a pearl away ...
After this speech, he stabs himself.
Othello rises in the end to the stature of the military commander and honorable man he is. He believed he killed Desdemona for honor's sake, but when he discovers he is wrong, he faces his fate unflinchingly. He shows no cowardice about committing suicide, which he understands is the only honorable way to atone for his crime. As he dies, we as an audience feel the loss of all this man could have continued to contribute to his society.