Harry Vallon was a New York City

New York City

New York is the most populous city in the United States and the center of the New York Metropolitan Area, one of the most populous metropolitan areas in the world. New York exerts a significant impact upon global commerce, finance, media, art, fashion, research, technology, education, and...

 gambler and mob informant.
He turned state's evidence and testified against the gunman in the murder of Herman Rosenthal

Herman Rosenthal

Herman Rosenthal was an American author, editor, and librarian.-Biography:Rosenthal was born in Friedrichstadt , Courland. He was educated at Bauske and Jakobstadt , graduating in 1859. In that year he translated into German several of Nekrasov's poems...

, and also against Charles Becker

Charles Becker

Charles Becker was a New York City police officer in the 1890s-1910s and who was tried, convicted and executed for ordering the murder of a Manhattan gambler, Herman Rosenthal in the Becker-Rosenthal trial. Becker was the first American police officer to receive the death penalty for murder...

, under a promise of immunity from the district attorney. He testified as one of four mob informants, along with Bridgey Webber, Jack Rose

Jack Rose (gambler)

Jacob Rosenzweig was an American gambler and underworld figure in New York City. He was one of several star witnesses in the Becker-Rosenthal trial, among these being fellow gamblers Bridgie Webber, Harry Vallon, and Sam Schepps. Rose's testimony was the most damaging because he directly...

, and Sam Schepps

Sam Schepps

Samuel Schepps , also known as Schapps, was a New York City mobster with the Monk Eastman Gang. Schepps ran gambling houses for Jack Zelig as part of his work with the gang.-Biography:...

 at the Becker-Rosenthal trial. Based upon his testimony, Charles Becker

Charles Becker

Charles Becker was a New York City police officer in the 1890s-1910s and who was tried, convicted and executed for ordering the murder of a Manhattan gambler, Herman Rosenthal in the Becker-Rosenthal trial. Becker was the first American police officer to receive the death penalty for murder...

, along with the four gunmaen involved in the murder, were convicted and sentenced to death. In 1936 he was threatened with rearrest in the case.