Given the importance and widely acknowledged greatness of this book, I would like here, in lieu of a “review” in the traditional sense of the term, to offer instead a few thoughts and comments toward a possible interpretation.
There is a great deal of Christian religious symbolism that runs throughout the book. There is, for example, the wonderfully succinct statement of the shortest chapter of the book, and perhaps the shortest meaningful chapter in all of English-language literature: “My mother is a fish.” The words of a child; simple, yet poignant and bursting with possibilities. What Faulkner has done in this single simple sentence is to turn the symbol of a fish, the ichthys of Christianity, a traditional symbol of the resurrected Christ, into a symbol of the finality of death, of the eternal absence of return. His mother is a fish because, like a fish, her eyes are lifeless, she has been gutted (metaphorically, in the case of the mother), and, of course, she flops around in the water when her casket falls into the river as they attempt to ford it.
It is in this Christian symbolism, I believe, that we can begin to arrive at a possible interpretation of the ostensible insanity of Darl. Darl is not insane in actuality, but is perceived as insane by the others because of his failure to conform to their expectations. He is different. He sees through things, he knows things, and he understands things. He is the only one of the members of the family that sees into the inner worlds of those around him, that is not entirely preoccupied with his own concerns. Dewey Dell even imagines that she has a conversation with him that takes place entirely in the realm of the mind. He penetrates her thoughts, he surpasses her objectivity.
And because he surpasses subjectivity he is frightening to the others. The rest of the family prefers their private obsessions. They do not want to be known. For this reason they have him taken away. They want to be away from his presence and the insight he has into each of them.
If all of this holds, Darl may be seen as a Christ-figure. He behaves in ways that do not meet other’s expectations and so makes them uncomfortable. He understands them perhaps better than they understand themselves, again making them uncomfortable. And he attempts to save them through a means which they do not understand and will not accept. In the end, they send him away because they want so badly to be out of his presence. There are, however, something (quite modern) fundamental differences between Darl and the usual Christ-figure. He is not killed and there is no resurrection; there is, therefore, no redemption.