The note could not be more vague. Presumably someone Rin wronged. It means something to her, but Mizu only knows it's something wrong because of the exercise. Kids get orphaned all the time. It isn't always because someone's being a murderous ass. It's likely not self-defense. You can feel bad about that, but you aren't going to blame yourself for the consequences.

"How were you in the wrong?" Mizu asks. She doesn't need the whole story but something. The scale is so small, however, when she must have created hundreds or thousands of orphans.