What drives a mob to climb several flights of stairs, break down a dormitory door and kill the young man inside? Secunder Kermani pieces together the last hours of Mashal Khan, the undergraduate beaten to death by vigilantes in April, 2017.It happened in the small city of Mardan, set on a fertile plain below mountains that form part of the border with Afghanistan. Until recently, this part of Pakistan was officially known as a “frontier”.Here, as in the rest of this huge Muslim country, blasphemy is a crime. And if the police won’t enforce the law, there’s a code. “If you have to kill someone as a punishment, do it in such a way that all connections to his brain are disconnected and there is no pain,” one local politician explained. “Just bury him afterwards.”Mashal Khan was not so lucky. His slow, painful death and subsequent mutilation was captured on mobile phones. The shocking footage spread quickly and reignited the controversy over Pakistan’s strict blasphemy laws.There have been rallies in support of the victim’s family. His grave is blanketed in tinsel and flowers from sympathisers. But there’ve been rallies for the alleged killers as well.The BBC’s Secunder Kermani is based in Pakistan and has gone to meet the families and friends on both sides of this story and asks, Who was Mashal Khan? And why did he die?