Part 217: (Hint 9)

"You mean someone killing the pair of them, then locking the door from the inside and hiding?"
"Yes. Since the master keys can't be used, the door must have been locked from the inside. So, it only makes sense if the culprit was hiding somewhere in Natsuhi's room."
"Right. That is weird. It says this in the narration: no matter how much time passed, Krauss and Natsuhi were the only ones who didn't return. And then it continues: everyone headed to Krauss's and Natsuhi's rooms together."
"If Krauss and Natsuhi were the only ones who didn't return, that implies that everyone else did return to the parlor."
"That's right. If the criminal locked themselves in Natsuhi's room after committing the crime, they shouldn't have been able to return. And yet, everyone except the victims did return."
"That would make this crime impossible for all the survivors... So, maybe we really should follow your theory of someone in the dining hall surviving and committing crimes later..."
"...Come to think of it, when I mentioned that theory, I was hit with a red truth."

My theory was that someone in the dining hall was alive and killed Krauss and Natsuhi. That theory was countered by this red: the culprit of the first twilight really did kill six people...

"Well, that red is hardly surprising. After all, the culprit went on to kill Krauss and Natsuhi, so they've killed a total of not just six, but eight people."
"...Huh...? Hold on... They went on to kill Krauss and Natsuhi... so, they really did..."