The following night Christine paces the drive before the portico of the Mannon house. Hazel arrives, having come at her request. Christine frantically confesses that she is unfit company for a young girl. She is old, ugly, and haunted by death. Hazel attempts to comfort her, promising to stay up with her all night if necessary. She leaves to tell her mother of her plans.
Alone, Christine sees Hazel meet someone at the gate. Orin and Lavinia appear. Orin reveals that they followed Christine to Boston, discovered her with Brant, and killed him. He shows her the few lines that announced his death in the newspaper. Christine sinks to the lowest step and begins to moan.
Pacing resentfully, Orin asks his mother to stop crying. He questions her: How could she have warned Brant against him? How could she have told him about their Island? Orin kneels beside her pleadingly, promising that he will help her forget and make her happy, that they can leave Lavinia at home and take that trip abroad together. In her father's commanding tone, Lavinia orders the crybaby Orin into the house. He salutes and obeys, remarking strangely that they should open the shutters and let the moonlight into the house. Father has gone.
Christine stares blankly ahead, her face a "tragic death mask." Lavinia declares that justice has been done. Christine glares at her daughter with savage hatred, rises, and marches toward the house. Lavinia asks what she is doing and how she can still live. Christine cackles shrilly, makes a motion to blot her daughter from sight forever, and rushed inside. Lavinia moves to follow but then determinedly turns her back on the house, standing like a sentinel.
From the street, Seth can be heard singing "Shenandoah" as he returns from the tavern. A shot is heard from Ezra's study. Lavinia stammers: "It is justice!" Orin's horrified cry rings out from the study. He rushes outside, blaming himself for her death. He decides that he must get her to forgive him. Lavinia silences him. She will help him to forget.
Singing, Seth comes from the drive. Lavinia orders him to tell Dr. Blake that her mother has killed herself in a fit of insane grief. Dumbfounded, Seth assents. Lavinia stiffly turns and follows Orin into the house.
In Mourning Becomes Electra you write: "Oedipus was the Theban king who unwittingly killed his father and MURDERED his mother." [Emphasis mine].
It should read: "Oedipus...MARRIED his mother!"
(Oedipus' mother Jocasta did commit suicide after learning her lover was her son. Oedipus however did NOT "murder" her.)