Vardaman and Darl go out for a walk at night to hear Addie talking from inside her coffin, as Darl assures Vardaman that he can hear Addie speaking to them. Darl explains that Addie is calling on God, in an act of Christian charity, to rid her from the sight of man. After they "listen," the two brothers return inside to check on Cash. Vardaman states that he saw something that Dewey Dell told him not to talk about—something that concerns Darl.
While Darl never first-handedly justifies burning Gillepsie's farm down as an act of Christian duty in his narrations, he does prepare Vardaman for this rationale by telling him to listen as Addie calls on God to remove her from the sight of man. Darl's trick is stealthy, and reveals his awareness of the way in which religion itself uses language to convince believers to act in particular ways.
Dewey Dell and Vardaman go to sleep outside on the porch, from which the barn is visible. Vardaman notices Gillepsie's son help Jewel, Darl and Anse move the coffin from below the apple tree outside indoors to the barn. Vardaman, as he promised to himself, goes to see where the buzzards stay at night. After doing so, Vardaman evidently spies Darl setting fire to the barn and repeats, "I saw something that Dewey Dell told me not to tell anybody."
Additionally, Darl is aware of how his family and others around him rely on the framework of religion and notions of duty to structure their actions and thoughts according to a moral compass. Thus, he utilizes this awareness to convince Vardaman that his action is in fact heroic. And in fact, there is something heroic in Darl's attempt to put an end to the ridiculous quest the Bundren's have embarked on.