While it’s clear that Noboru’s complaints about Ryuji stem from his own personal resentment—and not from Ryuji breaking any absolute moral principles—he still tries to give these complaints an air of objectivity. He sees Ryuji’s smile and wet shirt as crimes because they represent Ryuji deviating from the ideal of masculine heroism that Noboru has projected onto him. In this chapter’s closing lines, Noboru looks through the peephole just to prove that he hasn’t lost the power to do so. But he sees darkness, which the narration associates with both life (“alive with jostling particles”)
and death (“the inside of a large coffin”). Specifically, Noboru knows that the particles in his mother’s room are alive, but cannot see them, which is why the room reminds him of death. While this may or may not represent Noboru’s death, it certainly
does represent his powerlessness and loss of insight—meaning his sudden inability to understand the order of the universe, which always seemed clear to him in the past.