Who is Japheth Dury/John Beecham from The Alienist and what is their importance?

Japheth Dury is the unwanted child of Victor Dury and his wife. On the night Japheth was conceived, Victor brutally raped his wife, and Japheth became a living reminder of that night, as well as the object of his mother's anger and frustration. Japheth was later raped by a male friend of his brothers, and a short time later, Japheth's parents were brutally murdered. Japheth disappeared.

John Beecham is a soldier, who Kreizler's discovers was born in the same town as Japheth Dury. John was sent to an asylum in Washington DC after he was caught brutally stabbing a protester during the Haymarket Riots in Chicago. Beecham then made his way to New York City, where he began working with the Census bureau. In the end, Kreizler and his team establish that John Beecham and Japheth Dury are the same person, and that they are the serial killer who has been killing boy-prostitutes.