-=(blue)The culprit is Finn.
He saw the cabin while he was looking out from the balcony in the early part of the evening.
He left his door cracked so that he could hear when the others came up the stairs.
He killed Don and Carl and left them in their rooms. Don is dead in the shower. Carl is dead in bed, but that's not discovered as the light is not turned on when Amanda and Gabriel go into the room.
Bill is killed elsewhere in the woods and his body is hidden.
Eiko is killed and placed in the cabin.
The cries are from a frightened and hurt potoo, possibly hurt and scared by Finn but otherwise unrelated.
Finn returned to the cabin, killed Amanda, set the cabin ablaze, and dumped her body in the lake, before leaving in his car.=-
Sayo, otherwise known as Kanon/Shannon/Beatrice, was planning on pressuring folks into solving the riddle of the epitaph that night. To that end, one room and three doors were indeed prepared as per Let the Magic Rise, though only Eva's chain lock preparation ended up being used by the culprit. Jessica, meanwhile, spoke to Kanon, and together they hatched a plan to fake murders as the Golden Witch, Beatrice. Jessica wrote the letter found at dinner, and Kanon placed it. However, once real murders started happening, Jessica obviously didn't follow through with her plan.
Krauss, his mind finally snapping under the strain, began his rampage. He started with the wife who he felt constantly belittled him, as well as the servants who knew the secret of Kinzo's death. He shot Shannon in the chest, killing "Shannon" but due to her disguise allowing him extra protection, as well as a great deal of luck, Sayo only sustained some broken ribs and non-lethal bleeding. While Krauss was inspecting the bodies, Sayo slipped off his maid outfit to become Kanon and hide, dropping his hat in the process. Krauss discovered this, but didn't notice that Shannon's body, which would have been slightly covered by the open door, was now a pile of clothes. Krauss chased down Kanon and strangled him. However, falling victim to one of the classic misconceptions, he only strangled the boy to unconsciousness, then took his key. After Krauss had left, Kanon regained consciousness and debated what to do.
Krauss painted sigils on the doors and made the other guns nonfunctional. (Fun fact, in my head they were music symbols, but I could never find good bloody-looking music symbols and I didn't want to try to make them).
In the morning Kanon made a gamble and faked being dead, and luckily Krauss was expecting himt to be dead didn't truly investigate. Krauss faked finding Kanon's key on him, not knowing the boy also had Shannon's key. Gohda went to get the others, and Krauss went to the basement, which allowed Kanon time to leave, lock the door behind him, and get into the parlor and get Shannon's outfit on again. The parlor was discovered by the rest, and while Sayo debated calling out Krauss for having lied about the key, in the dark he hadn't been sure who the murderer was or if they had accomplices, so he resolved to try to find out first.
For the second Twilight, Rudolf turned out the lights. He knocked out Eva and tried to knock out Krauss, then went to the kitchen. Kyrie knocked out Rosa and Hideyoshi, and went there to meet him. They had resolved to hole themselves up in there, safe. However, Krauss with his boxing training could take a hit better than the rest, and saw Rudolf leave. Thinking quickly, he grabbed a tarp and made it into the kitchen before they could lock the door. They tried to fire their guns, but as Krauss had disabled them before passing them out, nothing happened, and Krauss stabbed them, using the tarp to shield himself from the blood. He opened the outer door to let the bloody tarp get blown away by the wind, and used some wire he found in the basement to pull a lightened table over through the crack of the inside door to block it. He kept the wire handy.
For the fourth twilight, Kinzo was always dead, and Krauss had the key to the study at that point, so he let himself in and painted a sigil on the mirror. It was an actual truth that the sigil was painted on the door the body was in, as the mirror was a door to a secret passage to the guest house library (which he knew of, as he had been the one involved in making the guest house), and Kinzo's corpse was hidden in it. Sayo had been lost in the dark like everyone else.
For the next three, Krauss snuck back to the mansion through that secret passage. Hideyoshi was resting in the VIP room, near-unconscious from the concussion. He waited until Eva left him to get something (which she had to unlock the door to do) before getting in and killing him. He then used the length of wire from before, bending the end into a loop, to set the chain lock from outside. He then hurried to Eva's room, surprising her and using the wire as a garrote to strangle her before she could shoot. He hung her in the closet. He was going to leave the chain lock undone, until he realized this room's lock was strangely longer (thanks to Sayo's separate preparations for a plan that never took place.) Gohda was by himself and short work was made of him. Once everything was set, Krauss returned to the guest house, called George from a different phone, and put on a reasonable-enough impersonation of Hideyoshi that George rushed off to find the bodies. Sayo had not approached the three because he had no evidence and they were likely to shoot anyone they weren't expecting. Afterwards Sayo was certain Krauss was the only culprit, but needed a plan to take him down.
Eighth twilight Krauss snuck back through the passage and shot George in the head, then returned to the guest house. It all happened too fast for Sayo to intervene, and he wasn't going to be able to take an armed Krauss on his own, so he hid again.
Krauss returned to the guest house, and Jessica stormed off. This was good, and gave him the chance to pick off the remainder one at a time by surprise.
In the mansion, Sayo met up with Jessica as Kanon and explained it all. By the time he had gotten her to believe him, Krauss had arrived. They confronted him about his actions. Krauss, insane, defended it, and went to shoot Kanon. Jessica dove in the way, taking a couple shots, before flying into a rage and beating her father to death. Kanon held her as she died. Sayo then contemplated all that had occurred, the truth of all the evil, and allowed the bomb to go off.=-
-=(red) Every time Kanon was found, it was Kanon's body being observed.
Knox's original 3rd is observed. Meaning, no more than one secret passage is allowed. There can be a secret passage from one place to another place, but there cannot be other passages connecting multiple places.
If the theory includes a secret passage, it must only be one. =-
-=(red)Rudolf was also killed in the kitchen. Rudolf is not the culprit.
The culprit never moved Kanon's body.
Eva's door was structurally sound.
Nothing relevant could be slid under the guest room doors.=-
-=(red)Rudolf and Kyrie were awake, alive, and unbound in the kitchen when they entered it during the blackout. They were on high alert and would have tried to shoot anyone who came in.=-
So why didn't they kill the culprit?