“The parents said they are especially frustrated because police told them that the people responsible for her son’s death are already behind bars, awaiting trial on two other murders.” from the article
You go with the cases with the best evidence. Even if you know you have the right person if you put them on trial with out enough to convince the jury not just the parents, if they are acquitted you can’t put them on trial again.
Read the article — the killers are already in jail.
You don’t announce it, they might put them in protective custody. If one were going to do something like that - and I’m not advocating vigilantism - but if one were so inclined... Best just to do it, and keep your mouth shut about it before, during, and after.