and the right to self defense didn't explain the "abandonment of a corpse." sure, they could've charged her with manslaughter or even murder, but why explicitly mention that she had to keep vigil around a corpse?! seems more like some religious thing in japan that found its way into the manga.
You are probably right about it being some sort of religious reasoning.
Just an example that may explain it... the death sentence in Japan is carried by hanging, and there is a WHOLE process to do it, too long for me to say every single detail so I will focus on what is relevant... the hanging is done by a contraption that is activated by a button, the button is in a separate room that has no window to the place of hanging, but... there are three buttons in that room, so three guards enter the room, they don't know which button is the real one, so they unlock the buttons each with a key, and press it at the same time.
Why is it done like that? Because in Xintoism, the religion Japan is most adherent to, murder is a capital sin, so even if the guards are only doing their duty as government employees, they still are committing murder in the eyes of their religion.
But there is a "clause" that saves it, is that as long as they don't "know" they are the killer, they are in the safe... yeah, don't want to hate others' religion but it is a stupid reasoning.
Anyway, thing is Japan has a peculiar relation with death, that is very much present in their laws.
One more thing, as I write this comment I remembered one thing... abandonment of a corpse being an infraction may have something to do with how corpses can be hotbeds for diseases, so abandoning a corpse may be considered a form of hazard dumping, this comes to my mind as I remember a lot of little details I know about Japanese culture and how century-old customs still affect their modern laws, I can be really wrong, but I do think there is at least a thread of truth in that.