 In 1984, McWilliams was jailed for fourteen years for his part in a shoot-out in a flat in the Lenadoon area of west Belfast in which a leading INLA figure, Paul McCann, and a Royal Ulster Constabulary (RUC) policeman died, but did not serve the full sentence.Seven years later, while still a member of the IPLO, McWilliams was sentenced to life imprisonment for the murder of Colm Mahon, a bar manager, on 15 December 1991, when Mahon asked McWilliams and his friends to leave the Frames Snooker Hall on Little Donegall Street, Belfast, on McWilliams' birthday.McWilliams later claimed that he did not carry out the killing and stated that he would name an alleged IPLO gunman who actually shot Mahon, but McWilliams never did name him.