Referred to as "a moral pyromaniac setting fire to all of reality," Iago of Shakespeare's Othello is not satisfied without taking down all that surrounds Othello; he simply cannot stop fighting his war of envy until all are destroyed. This is why Desdemona must die: Iago does not feel that he has completed his revenge until he has mutilated reality into complete nihility in his "genius of evil." Without question, Iago is pure villain. If he is to totally annihilate Othello, he must destroy completely all that he loves, as well; of course, this revenge on such a personal plane renders the tragedy all the more complete and malevolent. Without subplots and no comic relief as in Shakespeare's other tragedies, the plot of Othello is directed solely by Iago's evil intentions that include the destruction of Othello and Desdemona both.
In Act I Scene 3, Desdemona claims that hers and Othello's is a marriage of souls:
I saw Othello's visage in his mind,
And to his honors and his valiant parts
Did I my soul and fortunes consecrate. (1.3.249-251)
Further, she and Othello and Cassio are all deceived by Iago, calling him "honest" while he delights in his deception and plots to exploit Desdemona,
So will I trun her virtue into pitch,
And out of her own goodness make the net
That shall enmesh them all. (2.3.323-325)
and create doubt and suspicion in Othello,
....he echoes me
As if there were some monster in his thought
Too hideous to be shown. (2.3.107-109)
While Iago continues his diabolic plot:
Oh, beware, my lord, of jealousy,
It is the green-eyed monster which doth mock
The meat it feeds on. (3.3.167-169)
Then, in Act IV, Iago's duplicity and subterfuge continues as he convinces Othello of Desdemona's infidelity by creating situations which can be misconstrued. He even gets Othello so enraged that he orders Iago to give him some poison, but Iago urges Othello, “Hang her! … chop her into messes, and poison her” (4.1.191)
This vituperation of Iago demonstrates his hatred and his desire to destroy everything and everyone connected to Othello. Thus, Desdemona's death is in line with Iago's villainous nature and the depth of his treachery.
It seems as if the whole point of the play is that a man can be driven to kill the thing he loves best in the world. By far the most moving scene in the entire play is in the final act when Othello is loving and killing Desdemona at the same time. He calls her, "Thou cunningest pattern of excelling nature," and tells Emelia:
Nay, had she been true,
If heaven would make me such another world
Of one entire and perfect chrysolite,
I'ld not have sold her for it.
If Desdemona had not died, then Othello would not have committed suicide. This would be like Antony and Cleopatra or Romeo and Juliet surviving in their respective plays. Shakespeare liked to end his tragedies with multiple deaths because these have a stronger emotional effect on the audience. In Hamlet, for example, Laertes, Hamlet, Claudius, and Gertrude all die within moments, and meanwhile an English ambassador arrives to announce that Rosencrantz and Guildenstern have been executed. Previously Polonius and Ophelia had died. Elizabethan audiences must have liked this kind of carnage. We don't like to see Desdemona die--especially when the poor girl was totally innocent and full of love for the man who killed her. But that's what makes tragedy so tragic. Bad things happen to good people. Life is full of tragedy. Everybody has to die sooner or later. Happy endings belong in comedies or melodramas. In tragedies things happen that we wish hadn't happened. We don't like to see Ophelia die in Hamlet or Cordelia die in King Lear. We don't like to see young Juliet die in Romeo and Juliet.
Othello's killing of the wife he loved has a universal significance or symbolism. Othello kills Desdemona because he loves her so much that her apparent betrayal maddens him. Oscar Wilde pointed out this seeming paradox in "The Ballad of Reading Gaol," written serving a two-year prison term at hard labor. This is a bitter truth which many of us can relate to.
Yet each man kills the thing he loves By each let this be heard, Some do it with a bitter look, Some with a flattering word, The coward does it with a kiss, The brave man with a sword! Some kill their love when they are young, And some when they are old; Some strangle with the hands of Lust, Some with the hands of Gold: The kindest use a knife, because The dead so soon grow cold. Some love too little, some too long, Some sell, and others buy; Some do the deed with many tears, And some without a sigh: For each man kills the thing he loves, Yet each man does not die.