Desdemona is murdered at the hands of her husband, Othello; he strangles her in their bed. Othello's mind has been poisoned by the manipulations of the villainous Iago, but ultimately, Othello's actions are his own. The play is one of Shakespeare's famous tragedies, and the tragic hero always has a flaw that is both his own undoing and the cause of the suffering of others. Many have argued that Othello's tragic flaw is his jealousy, having killed his wife because he believed her to be unfaithful.
Othello is not the only character to experience jealousy. This theme begins with Iago, who is angry that Othello, general of the Venetian army, has promoted an inexperienced young man named Cassio. Iago believes that he should have received the promotion to captain, and he decides to avenge himself against Othello and Cassio both. Masquerading as Othello's friend, Iago sows seeds of doubt throughout each scene in the play, convincing Othello that Desdemona and Cassio are having an affair. Iago is a master manipulator, planting false evidence and spreading misleading gossip until Othello is persuaded.
Iago clearly has ill intent, and his many asides to the audience throughout the play highlight his exploitation of Othello:
Cassio's a proper man: let me see now:
To get his place and to plume up my will
In double knavery—How, how? Let's see:—
After some time, to abuse Othello's ear
That he is too familiar with his wife.
He hath a person and a smooth dispose
To be suspected, framed to make women false.
The Moor is of a free and open nature,
That thinks men honest that but seem to be so,
And will as tenderly be led by the nose
As asses are.
I have't. It is engender'd. Hell and night
Must bring this monstrous birth to the world's light. (1.3.353-365)
There are a number of factors at work here. Iago determines that Cassio's good looks will stir Othello's paranoia if it's suggested that Desdemona is spending time with the young man. That will not be enough on its own, so Iago later spreads stories about Cassio crying out Desdemona's name in his sleep. He also makes it seem like Desdemona gave to Cassio the precious handkerchief that Othello gifted to her.
Further, Iago recognizes that Othello has faith in people, and he doubly exploits that: Othello will trust Iago's word as a friend and fellow soldier and, if he believes that Cassio and Desdemona have betrayed his trust, he will be outraged at such an infraction against his values. Thus, the treacherous Iago perverts Othello's supposedly trusting nature so that he believes the liar, Iago, and doubts the loyal, Desdemona and Cassio.
But is Othello truly a trusting man if he can be turned against his own wife and captain? Iago preys upon Othello's insecurities in convincing him that Desdemona would betray him for a younger, more handsome man, but this speaks as much to Othello's own flaws as it does to Iago's duplicity. While he has been led by Iago to believe that his wife is having an affair, Othello is the one who decides that such an act justifies his murder of her.
Othello can't even claim that a murderous rage overtakes him and drives him to a crime of jealous passion. He kills Desdemona in a calculating and deliberate fashion. He soliloquizes as he approaches her sleeping form, telling us of his plan:
It is the cause, it is the cause, my soul,—
Let me not name it to you, you chaste stars!—
It is the cause. Yet I'll not shed her blood;
Nor scar that whiter skin of hers than snow,
And smooth as monumental alabaster.
Yet she must die, else she'll betray more men. (5.2.1–6)
He has thought this through, and he has decided upon his action. When Desdemona wakes before Othello can commit the deed, she pleads her innocence, but he chooses to reject the word of his own wife. "Guiltiness I know not," she declares, yet Othello commands: "Think on thy sins" (5.2.44–45). Othello insists that Desdemona gave Cassio the handkerchief, precious to Othello because it was his mother's and given to Desdemona as a sign of his love. It is a symbol of her supposed sin. When Desdemona denies this and asks him to send for Cassio to confirm their innocence, Othello refuses her. He is determined to see her guilty, and he denies Desdemona her voice, both in disbelieving her and strangling her until she can speak no more.
Othello behaves out of insecurity, jealousy, and misplaced trust. He is manipulated into trusting Iago's falsehoods, but he chooses to distrust his wife. Furthermore, Othello is the one who chooses to murder her; the responsibility is his own.