That's how the law would treat an adult. "That's not defending your home, that's murdering a man who isn't a threat" is what the adult would be told. They'd want a very good lawyer, they also would have wanted to aim better so there was only their side of the story

We don't apply the law like that against children* because children don't have the ability to control themselves or think before acting that adults have.

Have a bit of empathy for the kid.

*Except when it's very newsworthy

What sort of argument is that. Dude broke into a house. At that point he is a threat and doesn't magically turn into a harmless person just because they turn around. They might just be jumping into cover to get their own gun. They might be running to a second burglar for help. The first rule in any emergency situation is your safety is paramount. Yours alone. A firefighter won't jump into a burning building to save a puppy, a medic won't risk his life to constrain a madman with a knife. They are there to do a specific job - and heroics only looks good on TV or in movies.

Judging situations in hindsight is always so easy. Being in that situation is something else entirely. But it is better to be judged by twelve than carried by six. Again, your safety first - don't administer aid on a running highway, don't get closer to burning cars, don't try to "save" someone getting beaten up. Don't be a hero.

Did you know that people who run away can also run back? People under stress don't have the foresight to know if the threat is really over.

Discouraging shooting at fleeing people is good, but there will be situations where fleeing doesn't mean the end of the threat so we can't say it is always wrong. Like if someone said they would be back (not the case here), I would sure cut the defending person slack for shooting the fleeing person who threatened future violence.