Spoiler warning: Spoilers for the complete film or season named below.
Nile killed Madison, and Nina’s recording of his confession helps expose him. The Beast in Me then follows his punishment beyond the arrest: he dies in prison, while Aggie has to consider her own attraction to a story of revenge.
Why did Nile kill Madison?
She was assisting the FBI investigation into his business. Nile discovers that she is the source and kills her; his father and uncle conceal the body within the Jarvis Yards development.
That concealment makes the project more than the setting of a financial dispute. The public image of achievement literally rests on hiding what happened to the woman whose disappearance has been explained away.
Aggie approaches him as a writer carrying grief of her own. Claire Danes’ official interview discusses the uneasy relationship between author and subject. The investigation can produce a book, but it also pulls Aggie into the struggle over whose version becomes authoritative.
How does Nina get him to confess?
She confronts Nile with the evidence about Madison. His anger produces an admission she records. Once the recording is revealed and Rick cooperates with authorities, Nile can no longer rely on his family’s protection to sustain innocence.
Nina’s action matters because the evidence comes from someone inside the relationship. Information Aggie could gather had to cross a wall of denial before it could become a public account.
The confession does not mean Nina’s earlier position was simple. The finale also leaves her with a child whose future she cannot explain by treating Nile as an event safely confined to the past.
Who kills Nile, and what does Aggie make of it?
A fellow prisoner kills him in a final intervention arranged by Rick. Aggie’s book is published, but she questions the seductive idea that retaliation can draw a clean boundary between innocent people and monsters.
Nile tries to implicate her morally in his killing of Teddy. His accusation does not make her his legal accomplice; it presses on the anger and wishes she has had to confront.
Aggie’s progress depends on recognizing those feelings rather than assigning everything ugly to the man she investigated. Accepting them is different from endorsing his attempt to distribute responsibility for his crimes.
The final image of Nina with her baby extends that concern into another generation. Nile’s story has ended, but the people who lived beside him still have choices to make about what they will repeat, conceal or refuse.
Sources: official ending breakdown; official interview or additional context.