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.