The reason for John to faint at the end of the story is his shock provoked by the wife’s mental state. He prescribes the “rest therapy” to eliminate any distressing events that could worsen his wife’s depression. But this approach only leads the main character to insanity, which John observes in full glory in the last chapter.
John, The Yellow Wallpaper’s antagonist, is an abusive husband of the nameless narrator. As a caregiver, he prescribes his wife the “rest therapy.” It eliminates distressing events from the patient’s life. But it only worsens her mental health. The narrator is confined to their home and the company of a controlling family. She spirals out of control by obsessing over the yellow wallpaper. At first, the narrator finds it disturbing and asks John to remove it. He disagrees, implying that if he fulfills her wish, he would also confirm her paranoid concerns.
The main character fixates on the paper in secret. Ignorant John acknowledges it as an improvement. During the last scene, the protagonist reaches the peak of insanity. She attempts to free the imprisoned woman behind the wallpaper. Then John realizes his mistake. When he opens the door, he sees the patient tearing the wallpaper apart, creeping, and crawling in the room. It comes as a shock to him since John was sure that his wife’s mental state was improving. He faints, which also symbolizes that he gives up his power over the woman.