I thought it was just presented for the legal conflict making a good story on a show like “Law and Order”, which generally focuses on cases that are legally or forensically challenging in order to get its plots.
As for the true crime shows with kid killers, I don’t think true crime viewers get vicarious thrills from watching murder stories. I know I don’t. They’re mostly just interesting insights into the human condition. I think we’re all interested in knowing what causes someone to kill, especially when it’s someone who seems outwardly pretty normal or unlikely to be violent. And perhaps what we, individually or as a society, can do to prevent murders and protect others who need protection. I personally find it interesting to see that some kids who kill are clearly psychopaths from a young age, others appear to have mental illnesses that either weren’t treated or got the best of them, still others were abused or neglected and eventually took it out on another, and then there are the ones who seem to have just made terrible errors in judgment.