Hilliard's socialist beliefs overcame his religious calling; he abandoned his family and ministry and went to London, where he joined the Communist Party of Great Britain and returned to journalism. He subsequently joined the Communist Party of Ireland, becoming an atheist as well as a Marxist. He joined the International Brigades fighting for the Second Spanish Republic, either the Connolly Column of Irish volunteers or the British column. A comrade-in-arms recalled that he ostentatiously parodied the sign of the cross by reciting "In the name of Marx, Engels, Lenin, Stalin, Stakhanov, Dimitrov, the Party Line" while tracing the hammer and sickle with his hand. Hilliard was one of four riflemen in the rearguard covering the Republican retreat after the Battle of Jarama. All were killed, Hilliard dying of his wounds at Castellón de la Plana five days after being shot. Hilliard is among the combatants namechecked in Christy Moore's song "Viva la Quinta Brigada" about the Spanish Civil War and in Blanaid Salkeld's poem "Casualties", writing "That Hilliard spilled his boxers' blood/ Through Albacete's snow and mud/ And smiled to Comrade Death: Salud!