Father Aloysius Walsh spent the last years of his life painstakingly collecting evidence of murder - mass murder: a year-long killing spree of unparalleled savagery that blighted Ireland's borderlands at the end of the 1970s. Pinned to his bedroom wall, a macabre map charts the grim territory of death - victims, weapons, wounds, dates - and somehow, amid the forest of pins and notes, he had discerned a pattern. So why did he deliberately drive through...