I really like Kanz's theory, but I find it unlikely that the killer murdered Ryoko right in the dorm hallway, with a dozen potential eyes surveying the scene. We do need to explain, however, why the killer moved the body to the laundry room (assuming they did). Here's what I think happened.
I think the killer actually murdered Ryoko in their own room. Let's recap:
- Ryoko went out of her room towards the bath/on the 2nd floor. One or the other doesn't matter. She's crawling back to her room and happens to pass over the culprit's room, also dropping a bullet in Spiral's room on her way. That shows that she had her gun with her (this is important).
- Either the culprit or Ryoko tries to seize the occasion and off someone else. Ryoko has her gun: she fires a shot (which Spiral hears) and misses. Murderer grabs their knife and stabs Ryoko in the heart. He stashes her body in their bathroom for the time being.
- Killer fears for his life and is anxious to avoid a second room search. The following night, he then surveys the scene and decides to stash Ryoko's body in the laundry room when the coast is clear.
Only thing that we still need to explain is the presence of the fat suit with Ryoko. My guess for now is that she had it with her when crawling (in case she saw other people and didn't want to admit to not being fat). That would also explain why she was clumsy enough to drop a bullet in Spiral's room (assuming it was an accident).
Reposting my theory.
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When you have nothing to say, quote yourself ~ Solfadore