His final interior monologue is one of the most terrifying representations of insanity in all of literature. But it also seems to be a dramatic change, a collapse brought on suddenly by his family's betrayal, rather than the inevitable end of a gradual process. His musings on the instability of identity have degenerated into a loss of identity. To return to Cash's thoughts about sanity and insanity, the relativism of Cash's analysis is an important element of Faulkner's modernist experiments. More than James Joyce or Virginia Woolf, Faulkner's experiments more self-consciously emphasize the lack of an objective vantage point.
Truth emerges from a fragmented narrative.
Cash's observations about insanity drive home the point. Insanity is as much a matter of social convention as anything else. The others continue with life. We hear no more from Jewel, though from Cash's narrative it seems that Jewel took some satisfaction in Darl's institutionalization. Dewey Dell is absorbed in her own problems.