I wondered about this too... I mean how likely is it that he murdered a child when he was just 13 and then never committed another crime? Possibly I've watched too much Criminal Minds/ CSI but someone like that doesn't usually give up after the first time
Either they've not been caught for anything else or get sent to prison for a subsequent offence, which is what happened in the Clare Tiltman case - her killer was finally convicted after 21 years, but he'd been in prison for another offence for nearly 20 of those.
Poor Rikki, he had a pretty awful life from what I've read