The following describes the truth of what happened behind-the-scenes in End of the Golden Witch.
Culprit: Sayo Yasuda
Accomplices: Everyone except Natsuhi, Krauss and Erika.
Before the murders: Erika Furudo arrived on the island and met Maria in the garden. Sayo's goal had already changed in this episode, and had long before the family conference called Natsuhi as "the man from 19 years ago." The intention was to get revenge on her, since all that she went through was because of Natsuhi's past actions that resulted on her injury. Sayo easily bribed the adults with gold and the promise of making Natsuhi confess to hiding Kinzo's death. She herself continued making the calls to Natsuhi. Sayo prepared a murder prank to fright Natsuhi, having the adults convince Jessica, George, Battler and Maria that they were playing a prank against Erika.
The first twilight: Battler pretended he found Rosa, Maria, George and Jessica dead and screamed upon waking up. Thankfully, Erika, who was in the room over, did not go check that the bodies were actually dead. Genji also pretended to be dead. Krauss was kidnapped by Sayo and supposedly brought to the secret VIP room. After calling Natsuhi and allowing her to hear Krauss' voice, Sayo killed him. Later, the people pretending to be dead went hiding, then Sayo killed them all. During the previous night, Eva, to make the mystery game more interesting, gave Erika the idea of the seals, and Eva herself sealed the room she knew Genji would play dead in.
The knock and letter: The scene in which an "impossible knock" is heard, is from Battler's perspective. However it's later revealed in this episode that his perspective was not objective, also proven by the fact that he there saw Shannon and Kanon together. No knock existed, and no letter was placed. However, Kanon made the gesture of grabbing the letter from the floor, a letter he had on himself from the start, meaning he never actually "placed" it.
The second twilight: Sayo lured Hideyoshi to the room where Natsuhi was hiding, to continue with the prank, since he thought he would only have to play dead. Then, she killed him. The chain was not set and the door was never locked, however everyone else acted as if it was, tricking Erika. They did not realize Hideyoshi was actually dead, and Battler purposely stopped Erika from checking the closet, since he knew Natsuhi was inside.
Erika, having prepared everything to corner Natsuhi, presented evidences and even read Natsuhi's diaries to everyone. Natsuhi finally admits her sin against the 'the man from 19 years ago' and asks to be forgiven.
The rest is left unknown.