With the word "confess," Orin's tone instantly changes. He urges his sister to confess with him anew. Lavinia refuses. Orin calls upon their ancestors to haunt and hound her for a lifetime. Lavinia wishes for his death. Orin realizes that his death would be another act of justice and that Mother is speaking through Lavinia. He will find Mother again on the island of peace that is Death. He will kneel before her and beg for forgiveness and Orin convulses as if vomiting poison. He pushes Lavinia away with brotherly irritation; Mother is waiting.
Peter appears in the doorway. Unnaturally casual, Orin remarks that he was about to go clean his pistol in the study and exits. Lavinia stops herself from following and throws herself into Peter's arms. A muffled shot is heard.
Act III features Orin and Lavinia's final confrontation. Orin will speak the desire that binds them and their ancestors together. The pair's confrontation is preceded by Orin's last opportunity to escape the Mannon household, an escape made available to him by Hazel.
In this brief exchange, Orin entrusts Hazel with his precious manuscript. Interestingly, the manuscript appears here as not so much that which would bring the Mannon line to judgment as keep Lavinia in the Mannon home. As Orin tells Hazel, she is to read it if he dies or show it to Peter on the eve of their wedding if his sister successfully schemes to marry him. As we will see, these stipulations will enable Orin to keep his hold on Lavinia from beyond the grave.
Orin relinquishes the manuscript upon Lavinia's promise to do anything for him. Not content with the promise that she leave Peter, Orin confronts Lavinia, albeit through innuendo, with the incestuous desire that binds them. Orin proposes that they consummate their assumption of Father and Mother's places. For Orin, the consummation of their unholy union binds them together forever. Incestuous, cross-generational desire is displaced onto a sibling relation who members play Mother/Son, Husband/Wife, and Brother/Sister all at once. The conjured specter of Marie Brantôme—the wild, "animal-like" Canuck who wreaks havoc in the Mannon kinship structure—only lamely triangulates this incestuous pair. Though Orin considers Lavinia more a stranger than his sister and mother, his attraction to her, which reproduces the relationship between mother and son, runs in the family.
Lavinia recoils, perhaps as Christine did, at Orin's proposal. Then, at the word "confess," a break of sorts appears in Orin's speech, and he resumes his calls for atonement. Lavinia then wishes for his death, a wish Orin receives as if sent from Mother. Crucially this moment reveals that Orin's atonement is but another scheme to join the mother in the "secret world" of an illicit love affair. Orin clearly yearns for death at Mother's hands, convulsing symptomatically as if vomiting the poison that Christine put in his father's mouth. The punishment Orin so violently insists upon would allow him to go to Mother on Death's "island of peace." His death is not only the penalty for his crime but a means by which to be with Mother for eternity.