Darl is responsible for telling more of the story than anyone else, and we should sympathise with the burdens which he carries, as he is the character who has the most empathy for others. In this strange, dysfunctional family, we are not surprised that Darl suffers a breakdown as he takes on the problems of the other characters but has no outlet himself.
Darl has always been my favorite character in As I Lay Dying and I think it is because he is the only character with whom we might sympathize without pity. Vardaman is pitiful as is Cash. Dewey Dell, Jewel and Anse are too self-involved to garner any sympathy...
Sympathy is, perhaps, the defining characteristic of Darl Bundren. He cares for his sister, he mourns for his mother, and he understands the most enigmatic family member - Jewel. He is also the only family member who actively seeks to help the others to find comfort.