The shift in tone in act 5, scene 2 is marked. In scene 1, Iago brings to fruition his plan to have Roderigo attack Cassio; the scene takes place in the dark, and the two men fight in a kind of fog of confusion. The chaos is increased when Iago secretly stabs Cassio in the leg, and much of the second half of the scene involves the two men calling out for help before they can bleed to death. After the guard appears to find out what all the commotion is about, Iago appears again, pretending to be concerned with Cassio's wound, and kills Roderigo in what Iago hopes will seem a fit of vengeful rage. Othello's appearance on the scene and his calling Iago "brave" and "honest" serves to point out how deluded Othello is, even in this scene that shows, conclusively, the depths of Iago's villainy.
Scene 2, set in Desdemona's bed chamber, could not be more different in tone. The scene is quiet and tender, though extremely tragic. Iago's calculations have brought Othello to the point of murdering his wife. Othello, still in love with her, nevertheless is driven to kill her because he cannot stand the idea that she has cuckolded him. His innate nobility is tarnished by his jealousy and gullibility.
From the audience's point of view, these two scenes crystallize the emotions Shakespeare has been building for the entire play. Iago's villainy is made explicit and is revealed in scene 2; Othello's fate, at the hands of Iago's treachery, finally comes to pass; Desdemona's purity is proved even as she dies, as, with her last words, she asks to be commended to her "kind lord." Othello, for all his nobility, cannot overcome his own jealous nature. Iago, whose treachery is never explained and who remains alive at the end of the play, emerges triumphant—he may have to face torture, but his plans did succeed. The audience is left to ponder the nature of evil and its persistence in the world.