It’s a pretty simple concept. He is the one who performed the act. He is responsible.

So if someone calls up an assassin to murder another person, the one who ordered the kill gets off scott-free?

Unless you can show the adults deliberately taught him to murder, I’d say no.

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.

You argued that 4-year-olds don’t need supervision.

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.