Why is Darl's burning of the barn the climatic part of as i lay dying?

Realy this brings many questions about Darl all at the same time. Is Darl simply crazy? Is this his lashing out at the world? Perhaps he is putting his mother to rest? One wonders if the barn burning is Darl'a answer to all the pain that others seem to miss. In any case the burning certainly changes things.

Various interpretations are offered for Darl's act, but the "he's just gone crazy" interpretation seems unsatisfactory. Darl does seem to think that he hears Addie talking to him, or at least he says so to Vardaman: arguably, Darl might be speaking metaphorically about the body's need to be destroyed or buried, so that it will no longer be a source of disgust and loathing in others. Whether Darl believes Addie is speaking to him in a literal sense or not is really beside the point; his action is not dramatically out of synch with his behavior throughout the rest of the novel. In so many of his monologues Darl seems to transcend the division between literal and metaphoric; there is a powerful mysticism in much of what Darl says.

Undoubtedly, he is rattled by Addie's death in a way that the others are not. The humiliation of bringing the rotting body to Jefferson has clearly traumatized him. He is sensitive, and he believes that this act is an affront to his mother. For all of these reasons, Darl sets fire to the barn. Admittedly, it is not the most reasonable action. But burning the barn seems more the action of a desperate and traumatized man than a man who is simply insane.

This section is so climatic because the journey to Jefferson is not a time to make peace for Darl. His betrayal by his family will deal a killing blow to his sanity. After the barn burning he is institutionalized.