The following describes the truth of what happened behind-the-scenes in End of the Golden Witch.
Culprit: Sayo Yasuda (true culprit), Natsuhi Ushiromiya (accused)
Accomplices: Battler Ushiromiya, Genji and all family adults (except Krauss and Natsuhi).
Before the murders: Erika Furudo arrived on the island and met Maria in the garden. Sayo couldn't deliver the witch's letter to Maria. She changed her plans, with the sole objective of getting revenge on Natsuhi. Sayo easily bribed the adults with gold and the promise of making Natsuhi confess to hiding Kinzo's death. Sayo talked with her accomplices, who pressured Battler to make the calls to Natsuhi. Sayo prepared a murder prank to fright Natsuhi, having the adults convince Jessica, George and Maria that they were playing a prank against Erika.
The first twilight: Battler pretended he found Rosa, Maria, George and Jessica 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 somehow killed them all.
The second twilight: Sayo somehow lured Hideyoshi to the room where Natsuhi was hiding. Then, she killed him.
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 asked to be forgiven. Battler seems to regret being an accomplice.
The rest is left unknown.