Henry Russell was a Field Marshal assassinated by Thomas Shelby on behalf of the Crown, under the supervision and orders of Winston Churchill and Inspector Campbell, as well as Irene O'Donnell and Donal Henry, representing the British Government and the Irish Republican Army respectively.
Russell owned a house in Belgravia, which was the original site of the planned assassination on him, specifically for Russell to be shot in his own bed. An explosive device was placed into his house, through his letterbox, by John Shelby. This causes the planned site of his assassination to change to the Epsom Derby, to ensure the safety of Thomas by allowing him to assassinate Russell in a place away from the police.
At the Epsom Races, Russell is lured into a quiet area by Lizzie Stark, who represents herself to him as a prostitute. Despite Lizzie's clear objections, Russell attempts to rape her, but is interrupted by Thomas Shelby, who aims and pulls the trigger but his gun jams. Russell fights Thomas, but in the end Thomas shoots him in the head using Russell's own gun.
Russell's death is a political move, but he also had several crimes to his name, including being responsible for unspecified attacks in Cork.