Othello is a veritable wreck. He entreats the stars to look on his deed as punishing a crime, not as though he's committing a crime himself – it's important to him to declare that he is not the criminal of this whole sordid affair. He promises he'll not mar Desdemona's beautiful skin by cutting her up or anything – she'll be pretty in death.
Othello now notes the single candle he's brought into the bedroom, where he's found Desdemona sleeping. "Put out the light, and then put out the light," he says (5.2.10), looking at Desdemona. Othello muses that if he puts out the candle, he can easily light it again, but once he kills Desdemona, there is no way of getting her back.
Othello, during his moment, is almost overwhelmed by his love for Desdemona as he bends to kiss her. He says her beauty is almost enough to stop him from being an agent of justice. Notice how we said "almost" twice? Well, so did Othello, because feelings aside, he promises to kill her and love her after. He weeps over her, lamenting that he must kill what he loves most, alluding to the Biblical passage, Hebrews 12:6: "For whom the Lord loveth he chasteneth."
All this weeping and kissing and murderous plotting wakes Desdemona. Sleepily, she asks Othello if he's coming to bed. He asks her if she has prayed. "I would not kill thy soul," he says (5.2.32). (Think Hamlet again, like when Claudius almost dies.) Desdemona freaks out about the killing bit, and Othello doesn't deny, yes, I intend to kill you. He again asks her to think on her sins, so she can be absolved before she dies (and so go to Heaven instead of Hell).
Othello then brings up the handkerchief (which we've dubbed the handkerchief of death). He accuses Desdemona of giving it (among other things) to Cassio, and she tries to be very calm in convincing Othello that he has made a mistake. She denies ever having loved Cassio. Othello says that she's lying and that Cassio is already dead. Desdemona mourns Cassio, whom she says has been betrayed.
Still, she won't stop fighting for her life. Rather than come out with the story of having simply lost the handkerchief and try to clear up the whole matter, she begs to be banished rather than killed – to be killed tomorrow – to be given just half an hour more to live. But Othello is relentless, and smothers her with a pillow.
Emilia knocks at the door and calls out for Othello. Desdemona is still not quite dead, so Othello smothers her a little more. He gets lost in his thoughts about how he has no wife.
Finally inside, Emilia reports (wrongly) that Roderigo has been killed by Cassio, and that Cassio is wounded, but not dead. Othello is furious to realize that Cassio is still alive. But so is Desdemona – for at least three more seconds. "O falsely, falsely murdered!" she says weakly (5.2.117). Desdemona declares she is guiltless, but when Emilia asks her who murdered her, she refuses to name Othello. "Nobody. I myself," Desdemona says. Desdemona finally dies asking Emilia to give Othello her love (5.2.124).
Othello admits he killed Desdemona and tells Emilia she deserved it, and that honest Iago, Emilia's own husband, can tell her all about it. "My husband?" (5.2.144) Emilia keeps repeating, as the truth slowly dawns on her and she knows she married a jerk. Othello tries to get Emilia to calm down, mostly through his threatening looks, but she says she doesn't care about any harm he can do her. She continues to shout about the murder.
Soldiers and citizens of Cyprus crowd into the room, along with Iago. Emilia confronts her husband and asks if he told Othello that Desdemona was cheating on him. Iago admits that he suggested this, but then claims that Othello's own suspicion filled in all the blanks.
Emilia continues to lament Desdemona, and Othello continues to insist that Desdemona was false to him. Gratiano then announces that it's a good thing Desdemona's dad, Brabantio, died of heartbreak over her wedding to Othello, because he really wouldn't have survived traumatic event.
Othello continues to lay blame on Desdemona's foulness, and Iago keeps trying to silence the lamenting Emilia. Finally, the whole story clears up when Othello says that he saw Desdemona's handkerchief in Cassio's hand, and Emilia tells him that she herself stole the handkerchief and gave it to Iago.
With this small fact, Iago's whole story begins to unravel. Othello, furious, tries to stab Iago, but Iago avoids him, and instead stabs his wife to death (seemingly), and runs out. Montano bids someone to follow the villain, but only Othello and the slow, dramatically dying Emilia are left. Emilia wonders aloud about the meaning of Desdemona's final song, and she herself dies singing about willows, charging "the Moor" with some serious cruelty. Emilia dies next to Desdemona and tells Othello with her last breath that his wife was pure and faithful to him.
Othello finally realizes what he has done and curses himself. He finds a sword of Spain he keeps in the bedroom, and thinks suicidal thoughts.
Othello declares he's seen much worse in battle than this, but no man can escape his fate. He announces that no one has any reason to fear on seeing his weapon, as he's reached his journey's end. He then looks on his wife, murdered by his own hand, though innocent, he's now realized, and Othello weeps to end all days.
Lodovico, the wounded Cassio, and Montano enter with a recently captured Iago. Othello pronounces Iago to be a devil, and wounds him. Iago lets us know that he's not dead yet. Othello responds that he wouldn't wish such a peaceful fate as death on Iago (as Othello is preparing to meet such a fate). As Lodovico points out that grieving doesn't get you out of a murder charge, Othello says he can be called a murderer, but it's important to note that he did everything in honor, not in hate.
Othello demands that Iago tell him why he chose to torment him and ruin his life. "Demand me nothing. What you know, you know," Iago says. "From this time forth, I will never speak word" (5.2.303-304). Lodovico then brings out the papers that will clear the entire mess up. Roderigo, conveniently, had a bunch of mixed letters on his person that explain, in detail, Iago's complicity in all of these plots.
Othello, finally faced with Cassio, Othello learns that Desdemona was innocent and that everything that happened was part of Iago's scheme. And Roderigo lived long enough to reveal that his murderer was actually… Iago.
Lodovico orders that Othello be brought back to Venice for his punishment, and announces Cassio is to replace him. Othello wishes to say a word before he goes. He asks that he not be spoken of untruthfully, or in malice, as this tragedy is committed to history.
Othello declares himself the "one who loved not wisely, but too well," and then sums up the bulk of the play. In his final note, Othello pulls out a hidden weapon and stabs himself, declaring himself a circumcised dog. He has done an evil thing, but by killing himself, had conquered the villain (himself) and therefore become the hero of his own story. Othello kisses Desdemona's dead lips and then dies himself, a murderer, martyr, and lover to the end.
Lodovico tells Iago to look at his work: three innocent people lying next to each other, all destroyed by his scheming. Still, Iago keeps his promise and stays silent. Gratiano is to inherit all of Othello's worldly goods, and Montano is charged with punishing the wicked Iago. Sadly, Lodovico decides that all he can do is go back to Venice to share this tragic tale.