Darl thought that Cash would have told him that the family meant to send him to the asylum because Darl knows (as does Cash know) that there has never been any sense of antipathy or conflict between him and Cash. On the contrary, there was a sense of closeness. Darl apparently realizes that even Cash is unable to see the logic and necessity for his (Darl's) past actions. Darl's laughter is provoked because he can perceive the "metaphysical absurdity" of the situation while his brother, Cash, can only respond to the immediate act.
Why does Darl laugh? We have seen how Darl was able to see into the motivations of others, and now we must assume that he sees into all the motivations and understands how and why the others are afraid of him. He also understands that he is being declared insane so that Anse won't have to pay for the barn. These realizations leave Darl no alternative except to laugh or else actually lose his last bit of rationality. Thus at the end, the reader is once again reminded of the doubt surrounding Darl's supposed insanity.
Section 54, narrated by Dr. Peabody, returns us to the objective, outside view of the Bundren world. We have been so closely involved in the actions of the Bundrens that we need, at this point, an objective narrator to remind us of the total absurdity of the preceding actions. For example, the treatment that Darl receives, by analogy, is seen to be as absurd as putting the concrete on Cash's leg. Thus, we find that the outside narrator views Darl's insanity with skepticism. We are almost prepared to assert that the behavior of the Bundrens as seen by the outside narrator is more incomprehensible (or insane) than was the action of Darl.
