context: At the start of the film, in October 1986, a young woman is found raped and murdered in a ditch near a field. Soon after, another woman is found raped and murdered in a field. Local detective Park Doo-man, not having dealt with such a serious case before, is overwhelmed; key evidence is improperly collected, the police's investigative methods are suspect, and their forensic technology is near non-existent. Detective Seo Tae-yoon is sent from Seoul to assist them; their methods clash and he is unable to convince them they are dealing with a serial killer until his predictions of another murder come true. He realizes that the killer waits until a rainy night, and only kills women wearing red. A female police officer realizes that a local radio station is always requested to play a particular song during the nights the murders are committed.While Seo investigates and tries to piece together the clues, Park and his men beat confessions out of a local man found masturbating at the scene of one crime, and a mentally retarded boy whom they threaten to kill, going so far as to make him dig his own grave. Seo clears both of those suspects and follows a trail of clues to a factory worker who had only moved to the area a short time before the first murder. The detectives are unable to pin anything on him; when they realize that the retarded boy witnessed one of the crimes and try to speak to him, they frighten him so badly that he runs in front of an oncoming train and is killed.Finally, when yet another murder is committed and DNA evidence sent for processing in the United States comes back inconclusive, Seo's frustrations flow over and he snaps. He flies into a rage and roughs up the factory worker; only Park stops Seo from shooting the suspect.It's 2003 now and Park - now a businessman with a family - visits a crime scene - the first one, at a drain - from the case. He is interrupted by a little girl who asks him if something was there. He answers that he was just looking. The little girl then says that it seems so weird since she had just recently seen an another man with 'ordinary face' do the exact same thing. Park asks if she had asked him for the reason. She replies that the man said her that he remembered doing something there a long ago and so he had came back to take a look. Park is bewildered realizing that the man with 'plain face' must have been the serial killer from the unsolved case as the little girl innocently looks at his face.
question: What did they make the boy do?
answer: Dig his own grave