Manslaying is a peerless crime, because it is a crime defined by the nature of the creation itself, and not only by law. Therefore, even when the accused must be acquitted by law of Torah, he is still held responsible by our basic understanding of natural law and justice. That is only when we are convinced of the facts of the crime beyond a reasonable doubt, and the only reason to exonerate the defendant is a procedural one. The assumption that manslaying is a transgression of natural law, beyond being a crime against written law, is the basis of the huge extent of the liablity for causing death. Written law and justice always takes into account the criminal intention or negligence of the transgressor, and not only the deed itself.
Natural justice takes into account the very deed of manslaying.
The irreversible damage to the world by killing a person who was supposed to contribute his effort to the advance of the world, is the fact that matters. That is why the one who caused death by his action, even though found not guilty by written law, is still accountable for the death, and must atone for it. That is why K. is definitely responsible for the boy's death, and should atone for it, mainly by contributing as much as possible, in time, money and effort, to the prevention of such needless deaths.