I have a very odd prediction for this one.

First he dies,

Trying to protect the protagonist.

With the protagonist out of commission during the investigation, someone will have to take up the mantle for this chapter.

That being xem (did I use that neo pronoun right? Please correct me if I'm using it wrong)

Xe is protagonist for this chapter

The killer will end up being him.

Basically he was frightened and scared. Enough to snap and kill someone.

My prediction:

Victim: Shinjiro Takahasho - Ultimate Linguist

Blacken: Keiji Hatano - Ultimate Pyromaniac

(Keep in mind that this is my opinion)