Here's what I think... the killer knifed Carrie, and was trying to make a locked room out of the whole shebang, but then Koishi snuck up and grabbed the knife from them - she would have pretty good range with her tentacles, so she could easily flick it back to her and run away. Then the killer scooped the blood out of the ice cream, chased her with the scoop, and took the knife back after she was dead! Then the killer put them in the vending machine on the way back. Am I a genius or what?