I'm not empathizing with the suspect, nor morally equivocating. Our justice system generally is over-punitive, especially for minorities and poors and this kid just thrust himself into the system.
When we have a loss of life, we quite reflexively revert to "someone must pay" and that frequently leads to a rush to judgment. And that judgment often leads to an incredibly harsh sentence.
All crimes, at their basic level, are bad decisions. Some of those bad decisions are made by those with a long history of bad decisions from the trivial to the grave, in terms of morality and in terms of consequences. They're schizophrenic, psychotic, or psychopaths, or otherwise deeply damaged human beings. Others aren't, but due to certain circumstances of a possibly temporary origin, make these horrible decisions.
We know as a matter of first principles that those under 25 are prone to bad decisions, maybe not this bad, but bad nonetheless and frequently of a criminal dimension. That's why we have a juvenile justice system, in the first place.
Juvenile justice is often equated with a lack of consequences, but I don' think that's the general case. We tend only to hear about juvenile cases when the juvenile in question goes on to commit adult crimes, that is, they prove to be the "psycho" type prone to continued "epically" bad decisions.
One of the problems, to my mind, with juvenile justice is that it ends too early. Meaning two things: those with adolescent minds lose eligibility at 16; and possibly worse, the ability of the juvenile justice system to deal with juveniles ends at about that age, meaning it can't incarcerate or otherwise treat them beyond majority, regardless of when they committed their crime.
There are a ton of unanswered questions in this case that bear on what kind of bad decision-maker this kid was. They may have less bearing in adult criminal court, but could still be relevant to sentencing.