Spoiler warning: Spoilers for the complete television adaptation.
Abby kills Merritt because her pregnancy threatens the timing of the Winbury inheritance. The Perfect Couple hides a financial calculation inside a family apparently consumed by affairs, resentment and a ruined wedding.
How does the trust create a motive?
The sons receive their inheritance when Tag’s youngest child turns eighteen. With Will approaching that age, a new baby fathered by Tag would restart the wait and divide the money among another heir.
Thomas is in debt, and Abby is expecting their child. Merritt’s pregnancy therefore threatens a payout Abby regards as essential to her own future.
The important distinction is between knowing about the affair and needing its consequences removed. Several characters have reasons to be angry; Abby has a specific reason the pregnancy must not continue.
What links Abby to the killing?
She drugs Merritt’s drink and then drowns her during their swim. Her careful washing of a glass, despite usually expecting the housekeeper to do domestic work, becomes an early detail worth reconsidering.
The glass matters because it connects an ordinary action to an effort to remove evidence. The clue does not depend on Abby looking overtly sinister when the police arrive.
Her behavior toward the grieving guests works similarly. Participating in the family’s concern offers a position from which to remain near the investigation without appearing to stand outside it.
Why return to Amelia after the arrest?
The later meeting between Greer and Amelia at the London Zoo gives the bride a life beyond the wedding she never completed. Greer approaches her with a manuscript and an invitation.
That encounter does not turn the women’s earlier conflict into a misunderstanding. It lets them meet outside the family structure that required Amelia to perform an acceptable version of herself.
Abby’s crime is organized around securing her place within the Winbury fortune. Amelia’s final position is elsewhere. The contrast gives the ending a consequence beyond identifying the killer: one woman destroys a life to preserve access to the family, while another can begin to exist outside its expectations.
Sources: official ending breakdown.