I would consider some other charges, like assault with a deadly weapon, or attempted assault? I'm not sure what would apply and be fair.
My feelings are very similar to yours, but I can't see a plausible legal justification for any charge other than first degree murder with these facts. I wish there were another way to charge it.
When the killer squeezed off the kill shots, he was in absolutely no danger from anyone, so self-defense is not available. Insanity is a pretty hard sell in this case. So the main two affirmative defenses are gone.
Of course there are a lot of lesser included charges, like aggrevated assault, but are they appropriate for the execution slaying of a wounded target? I'd say no. But a jury looking at the case charged as murder one could decide the penalty should be scalable and vote a lesser crime. The danger is that most of the lesser charges can carry very light sentences. In my state, Florida, a judge has a range from probation to life with possibility of parole in twenty years as sentencing options for the most likely ones.
Probably the best hope for justice here is a civil claim of wrongful death. He might not do any jail time, but his pension and all assets would go the kid's families.
If I were on the jury I would vote for murder one, and if available, death eligibility. If one of the parents shot him on the courthouse steps, I would probably vote to acquit on the basis of temporary insanity.
Better a society that lets you off for killing for being pissed over loosing a child than a society that lets you off for killing for being pissed over a B & E.