Prosecutors on Wednesday played the videotaped confession of a 19-year-old to murdering his 4-year-old half-sister, making public the killer's motivations for the first time. Keith Randulich was an 18-year-old, academically successful senior at Lincoln-Way East High School in the Chicago suburbs, in May of 2009. On the night of May 22, with his parents out of the house and his brother playing a video game upstairs, Randulich took Sabrina Clement to the basement of their Mokena, Ill. home and cut her throat. Investigators found in the basement a collection of horror films, death-metal music, a rubber clown mask and what the Chicago Tribune described at the time as "disturbing writings." Randulich has said that he believed young Sabrina was being sexually assaulted by a family member, and that he took her life in order to save her from further abuse. The coroner found no evidence of sexual abuse in Clement's autopsy. Prosecutors are seeking a life sentence for Randulich, arguing the especially brutal nature of the crime. To bolster their arguments, they played the tape of his confession, given to police shortly after the murder. The Chicago Tribune reprinted excerpts from the interview: When Clement told Randulich she wanted to paint with him, her brother said he would, but asked her to lie on the floor first. He then knelt down, apparently straddling her. "I just said I loved her and I took out the knife and she smiled at me," Randulich told the detectives. "I made just a little cut in her neck and she screamed and said, 'Stop!'"