And what if noone was warm to him, who is at fault when the village burns?
Him.
It's a pretty simple concept. He is the one who performed the act. He is responsible.
I'd say the adults are.
Unless you can show the adults deliberately taught him to murder, I'd say no. If you can show they did that, they can join him in prison forever. But he doesn't get a pass.
I'm perfectly happy to blame the adults for a kid becoming a little shithead asshole. Not so much when the kid deliberately decides to murder someone.
You argued that 4-year-olds don't need supervision. Now you're arguing that 15-year-olds are incapable of being responsible for their own, deliberate actions; that their parents, guardians, or other individuals charged with supervising their behavior are responsible.
Hear, hear!