How does chapter 18 of Brave New World reveal John's character?
Mostly focus on this passage:
The Savage had chosen as his hermitage the old light-house which stood on the crest of the hill between Puttenham and Elstead. The building was of ferro-concrete and in excellent condition-almost too comfortable the Savage had thought when he first explored the place, almost too civilizedly luxurious. He pacified his conscience by promising himself a compensatingly harder self-discipline, purifications the more complete and thorough. His first night in the hermitage was, deliberately, a sleepless one. He spent the hours on his knees praying...stretched out his arms...in voluntary crucifixion, while he repeated, through clenched teeth "Oh forgive me! Oh, make me pure! Oh, help me to be good!"
John the Savage has asked to be allowed to go with Helmholtz to the islands, but he is denied because Mustapha Mond wishes to continue the experiment with the Savage. So, since Mond will not grant him exile, John chooses exile himself where he can be alone and experience what he calls "the slings and arrows of outrageous fortune," using the words from Hamlet. As his hermitage, John selects a lighthouse, an isolated spot that affords him the view of nature and solitude from the horrible masses of Deltas and others.
He comes to this area to be purified, to make amends to himself and to Linda; he wishes to "escape further contamination by the filth of civilized life." There he focuses on the "immensities of death and deity," regaining a sense of the spiritual. However, when the Deltas discover him and Darwin Bonaparte films him, John's life becomes part of a feely, and he is again exploited. His efforts at penance leave him bereft, and he still feels his sexual desire for Lenina when she arrives. For, even though he hurls his Shakepearean epithets at her and self-flagellates, his lust overcomes him and he falls into the orgy-porgy that the viewers begin.
John is sickened by his weakness in giving in to sins of the flesh. His efforts at self-penance have failed as he engages in the orgy. There is no penance left for him but self-destruction. Like the figure in voluntary crucifixion that he was previously, John hangs himself in his despair of freedom and purity, making himself an example of how no one can be "unstable" and survive for Mustapha to use for his utopian society.