Helmholtz and Bernard return. Mond is gone. They hear the Savage retching in the bathroom. He tells them that civilization and his own wickedness defiled him so he drank some warm water and mustard to make himself throw up, and thereby purify himself. Bernard and Helmholtz are shocked. Still, they say goodbye: they're heading to the island the next morning. Bernard apologizes for his behavior. The Savage says he asked if he could go with them, but Mond refused. Mond wanted to continue the "experiment."
John's use of physical self-purifying techniques is an effort to instill in himself the discipline to not succumb to the temptations of promiscuous World State culture. He's making a noble stand as an individual.
Some days later, the Savage settles away from any city, in an abandoned lighthouse. He brings a few supplies, determined to grow a garden and become self-sufficient. When he catches himself being happy, which he considers offensive to the memory of his unkindness to his mother, he whips himself.
For a man determined to make a stand against a totalitarian government that forces it's citizens to be happy, unhappiness is the only freedom.
Some Deltas passing on a nearby highway see him. The next day reporters show up. The Savage abuses them verbally and physically, and soon is left in peace. But one day the Savage has lustful thoughts for Lenina. He whips himself more viciously than ever, and a Feelie photographer who had been hiding nearby catches the whole thing on video. The next day a massive number of sightseers come to watch the Savage. They beg him to whip himself again.
John's agony, as well as his methods to keep himself from joining the happy Wolrd State culture, are just a curiosity to the World State citizens. They don't have the capacity to understand his deep and powerful emotions.
Lenina steps from a helicopter behind the crowd. The Savage rushes at her, screaming "Strumpet!" He whips her, and himself. The crowd goes into a kind of ecstasy. Someone chants "orgy-porgy." Soon everyone is chanting it.
John's emotions overwhelm the crowd. World State Citizens have been trained to let off emotion in just one way: Solidarity Service orgies.
The next morning the Savage wakes. He sees that the crowd has gone, but he remembers the orgy of the night before. When new sightseers arrive the next day hoping for a repeat performance, they find that the Savage has hanged himself.
For John, sleeping with Lenina is submitting to the slavery of happiness. He's lost his battle with the world state, so he kills himself. The World State continues on, another individual sacrificed to stability.