LogFAQs > #933508019

Hint 1: Find someone who can never be the culprit.

That isn't very useful. But it does confirm that it is possible to eliminate a few characters in this way first.

Natsuhi's room was sealed after the second twilight. Alive or dead, they could never leave the room because according to the red, the culprit didn't join them, they definitely sealed the room and it was never broken. Therefore, even if one or both is the culprit, they can only be involved in the first and second twilights. And the culprit of the first twilight must kill 6 people.

Furthermore - the red says that only servants can use master keys. So even if a family member lied about destroying the master keys, they cannot use it. On the other hand, it would be redundant for a servant to take a master key from a dead body.