Life, liberty, rights, and privileges can - and should - be deprived upon conviction of a crime.
Life can and should be deprived? That's barbaric. Every civilised country has abolished the death penalty. Heck even parts of the US managed to abolish it.
I summarily reject your suggestion that a 15-year-old is so lacking in their capacity for executive control that they can be excused of murder.
So you reject reality. Which explains a lot.
By all means, be warm to the kid. Until he starts setting people on fire.
And what if noone was warm to him, who is at fault when the village burns? I'd say the adults are. Punishing the kid is just them trying to cover up their own failures. A convenient scapegoat for their own failure to foster wholesome interactions.
So if someone calls up an assassin to murder another person, the one who ordered the kill gets off scott-free?
Adults have a duty to raise kids well, just as they have a duty to file their taxes. If they cannot do so on their own, they have the right to be helped along by the rest of society. And, really, even if not there's that other (more famous) African saying: It takes a village to raise a kid.
Consider the alternative, or, rather, that really seems to be what you're implying: That children are responsible for their own upbringing. Next up: Babies are expected to grow their own food. Your potted petunia is responsible for its own watering.
If they have shown signs of being violent to their peers, yes of course they need supervision. And so does our 15yold. But that doesn't mean that we pre-emptively supervise every kid that way they'd never learn independence, and thus never truly become adults, they'd just spinelessly bow to the next random person who passes as an authority figure.