So who exactly did they attack here?

Apparently it's Cirno, according to one of the tags, and the comments on pixiv.

Can nayone see (or translate) whatever is being written by the victim?
*it sounds like I got that from a detective show or something like that*

yeah, if I remember correctly, they usually write the name of their killers

Nah, she's still alive. What gets me is how there's apparently blood.

"Are there any bad kids here?"

Spirited away by the oni

They kill Cirno for FREEZING the underground.