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.
The principal action of Act V is Christine's suicide. Her exchange with Hazel, in which Christine implores her to keep her company for the night and bitterly reminiscences anew about once being as innocent as she, build the tension of the scene through oppressive foreshadowing and dramatic irony. Christine's pleas only forestall her coming doom.
Orin and Lavinia's arrival quickly bring her fate to its conclusion. Orin confronts his mother and flaunts Brant's death before her. Christine collapses in grief, her face becoming a "tragic death mask." Almost immediately Orin attempts to make reparation. He cannot believe his mother's grief, cannot understand that Christine could have truly loved Brant. Indeed, Orin is certain he can take Brant's place. They can leave Lavinia and the house behind them and flee to the Blessed Island themselves.
As Christine has repeatedly remarked, however, Orin is no longer her beloved. His abandonment of her under Ezra and Lavinia's orders made him no longer hers alone—Brant was his replacement. Lavinia, moreover, will not allow mother and son their "secret world" either, the pre-Oedipal Eden constituted in defiance of the Mannon line. Embodying her father's voice anew, she orders Orin to break his incestuous embrace and enter the house. Orin complies automatically, almost wistfully protesting that father has gone away.
Christine kills herself soon thereafter. O'Neill makes use of a device in his oeuvre, one that appears in The Iceman Cometh and elsewhere: the period of terrible suspense between a major player's decision to suicide and the final act itself. Lavinia refuses to intervene, planting herself like a sentry before the house. As if to push all doubts from her mind, Lavinia forcefully declares Christine's death, like Brant's, as an exercise in "justice."
Importantly, Lavinia does not seek the justice of state or juridical law. Though Christine's box provides her with the necessary evidence, recourse to this law would bring disgrace to the household. Instead, Lavinia demands the justice repeatedly exacted by the many Mannons before her: the justice of revenge. Unlike legal justice, this justice-by-revenge perpetuates itself cyclically across the generations as the wronged and revenged come to make their own demands for retribution. The demand for revenge returns in the following act with the guilty Orin, who will appear bent on atoning for his mother's "murder."
Though by now the inexorability of fate should be clear, Lavinia, standing as the house's grim, black-clad sentinel, the functionary of its ancestral residents, and guardian of its secrets, quickly arranges the repression all that has just ensued. Insisting that her brother be quiet, she promises to help the guilty Orin forget their crime. Immediately she moves to conceal the history of her mother's death. As she tells Seth, Christine has killed herself in a fit of grief over Ezra. Brant is reduced to a footnote in a newspaper—their affair has been erased from recorded memory. The Mannons' longtime servant knowingly colludes in this exercise in repression.
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.)