One week after leaving the borstal, Roberts was called up for national service and joined the Rifle Brigade (Prince Consort's Own), with whom he saw action during the Mau Mau Uprising and Malayan Emergency. Of his service in the jungle he said that this was where he learned to kill and that he had "personally killed at least four". Roberts has claimed that he reached the rank of Sergeant while in the Army although others have given his rank as Lance Corporal. Journalist and former armed robber John McVicar has said that Roberts "gloated" about his killings while in prison, and had "acquired a taste for killing prisoners war on the orders of his officers" in the Army.