Before we get too caught up on who the killer might be by trying to figure out Miyamura's motives or capabilities, what about after Miyamura and Jin were dead? Everything probably happened in the greenhouse, and the actual killer may not have expected to kill anyone.

The killer visited three places that we know: the river, the infirmary, and the furnace, where they burned something. It's possible there's another clue we haven't found yet.