When I read the main series, I can understand the perspective of an op hero who couldn't save his friends. In this spin off, it feels like one of those scenes where the MC's sensei/senpai/op commander/leader/king where we don't have the knowledge of Rimuru entirely, but we know his feelings. Its quite nice how they nailed the feeling of an "outsider" when there is already a story that provides insight of the "main character" of the series.