You got the purpose of juvenile justice completely wrong: It is focussed more on rehabilitation and less on deterrence than the adult one because juveniles are still way more formable. Psychologists will descend upon him, and they'll do the job his parents and neighbours didn't (or couldn't) do, a job which, at 15, noone is able to do on their own.

those determined to be fundamentally irredeemable.

That's vile. Of course they'll be unredeemable if you don't give them the chance to redeem themselves.

A youth justice system is for dealing with kids and teens who shoplift, or break noise ordinances, or run away from home, or abuse illicit substances, or any number of “boundary exploring” behaviors.

A youth justice system is not the appropriate venue for dealing with “kids” so lacking in moral fiber as to deliberately and maliciously kill another person.

If you're distinguishing by the type of offense instead of by age, you don't have a youth justice system, you have a minor offense justice system.
Distinguishing by the severity of the offense is already part of the justice system.
Youth justice systems explicitly consider the age and maturity of the offender, not just what they did.
Also I'm not sure why a 15-year-old is a kid in one of your examples and a "kid" in the other.

The tolerance we have for “youthful indiscretion” does not and should not extend to this degree of violence. A youth justice system is not an appropriate venue for those determined to be fundamentally irredeemable.

This is not about tolerating behavior, it's about reforming people to become members of society instead of lifelong burdens for the justice system.
Despite the severity of his action, brandishing kids as "irredeemable" not only throws away their entire future but also burdens everyone else with keeping them contained forever.
That profits nobody.