The following describes the truth of what happened behind-the-scenes in Legend of the Golden Witch

Culprit: Sayo Yasuda, dressed mostly throughout the murders as Kanon as she had Shannon "die" in the first twilight.

Accomplices: Genji Ronoue, Chiyo Kumasawa, Terumasa Nanjo, Eva Ushiromiya, Hideyoshi Ushiromiya.

Before the murders: Sayo, dressed as Kanon, played the part of Beatrice and gave Maria the letter. As Maria was used to Beatrice "possessing" servants like Kanon and Shannon, she accepted her as Beatrice. Sayo tempted Eva and Hideyoshi with the gold she inherited from Kinzo to help her in her murder game. Eva and Hideyoshi were not aware that Kanon had killed them for real (as seen in how Eva insisted to Battler that the victims had "makeup scribbled on their faces" instead of blood).

The first twilight: Kanon invited Krauss, Rosa, Gohda, Kyrie and Rudolf into the dining hall, where he shot the five of them. With the help of Genji, he moved the corpses into the rose garden storage room and locked them in. He notified Hideyoshi and Eva to pretend that Shannon's body was there when it actually wasn't, to fake a death for his other persona Shannon, and told them how the ring George had given her was. Previously, Kanon opened Natsuhi's room with a mastery key and saw the scorpion charm hanging inside. To create an idea of a "witch who cannot touch scorpion charms", like Shannon had told the cousins earlier, he closed the door again and splashed red paint on the door of Natsuhi's room to scare her and gave up on the original plan of killing her.

"Illusions to illusions. The corpse that cannot return to earth returns to illusions." - Willard H. Wright

The second twilight: Eva and Hideyoshi were the only potential victims for the second twilight since they were the only "two who are close" left, so they were chosen, and this eliminated two potentially dangerous accomplices. As Kanon, she knocked on the door of their room, claiming to discuss the next step of the scripted murder, having previously ordered them to eventually go to a guest room alone. She shot them in the head and inserted the stakes in the bullet holes. She pretended to have stumbled on the guestroom and cut the chain before finding the bodies, creating an effective closed room murder. The magic circle was quickly drawn while the others were nearby. 

According to Our Confession, the stakes were always pierced by shooting the body spot with a gun, removing the bullet, and placing the stake. 

"Illusions to illusions. A chain of illusions can only hold back illusions." - Willard H. Wright

The fourth twilight: Since Kinzo was dead all along, it was not difficult to plant a stake in his head and place his body in the boiler.

"Illusions to illusions. Let the man of illusions go to where he belongs." - Willard H. Wright

The fifth twilight: Kanon ran into the boiler room, pretending to have heard a door closing. Sayo then has an internal struggle over whether or not she should continue the murders, represented by "Kanon" yelling at "Beatrice" to stop the murders. She decided to go through with it anyway, effectively "killing" Kanon. Sayo then planted fake blood on her chest and a bloodied stake near it to simulate the death of Kanon. Nanjo, one of her accomplices, pronounced Kanon dead and everyone left the body. This allowed Sayo to move around freely while everyone else was cooped up in Kinzo's study.

"Illusions to illusions. The witch and stake of illusions can pierce naught but illusions." - Willard H. Wright 

The sixth, seventh and eighth twilights: Maria, Genji, Kumasawa or Nanjo (most likely Genji) left the letter on the table, hoping Natsuhi would kick them out and Sayo could continue with the twilight murders. Sayo then dressed as Beatrice, and murdered all of them except Maria, opening the lock of the parlor with her own master key. She then called the study and had Maria sing for them. Maria's account was instructed by Sayo.

"Illusions to illusions. Illusions are the blind girls's song. Illusion of a closed room." - Willard H. Wright

The ninth twilight: Sayo dressed as Beatrice, lured Natsuhi out with a letter that says "Ushiromiya Natsuhi-sama. Let's settle which of us is more suitable to be the family head. I will be waiting in front of my portrait". Thankfully for Sayo, she was the one who first picked it up and not one of the cousins. Natsuhi found her and challenged her to a duel, but unbeknownst to Natsuhi her gun had blanks, creating the illusion she had shot herself.

The tenth twilight: The clock stroked twelve, and the bomb killed the five remaining.