How about something a bit stronger.
The culprit is Lykos.
To create the disappearance at the ferris wheel, the culprit and Beowulf worked together. As any theme park employee would know, it is imperative that a ferris wheel is balanced. That is to say, people must be loaded such that the weight is equally distributed along the wheel.
Of course, Beowulf's request all but guaranteed that this would NOT be the case.
Once his cabin reaches the top, the accomplice enters the scene. He loads up the cabin just below the halfway point of the wheel--which is permitted by the previous red about venturing to the top of the wheel--with weight. It's likely they used a combination of their own body weight as well as another weight the accomplices had available to them: the bag of silver they'd stolen from the moon cult.
Using this approach, the wheel would rotate ever so slowly such that the two weighted cabins at 12 and 3 o'clock move clockwise. The accomplice can shift weight from the cabin they'd weighted into the cabin shifting to the new 3 o'clock position until Beowulf's cabin is within reach. Here, they unlock the bolt, freeing Beowulf.
If you want to be a stickler about how the top cabin was found locked, assume every cabin was locked beforehand, and only the cabin loaded up with silver is unlocked, then relocked as it that cabin "shifts".
Now for the apartment. We don't have much room to maneuver here. There really only seems to be one opportunity for the victim to enter here. Let's forget about the man caught on camera by Fenris: that's someone else, so Beowulf likely entered through an open window somewhere else in the building.
Our window of opportunity is the moment Wolfgang unlocks and opens his door. The blood trail in the hallway (the only part of it Wolfgang says is old/dry) was created earlier by Beowulf, from his leg wound. After Wolfgang opens his door, he sees blood going into his entryway, which is the first suggestion that the victim has already entered the flat before then. This is simply impossible, so I have to imagine Wolfgang is wrong here: he's in a bad state, so it's not hard to imagine that there were spots in his vision, making it look like the trail continued inwards.
So at this point, the victim rushes in, chains the door, and dies. But now we have another problem: death was instant, wasn't it? So who delivered the killing blow, and how did they escape?
This is just a trap. The narrative suggests that death was instant by appealing to the police's conclusion. But this simply cannot be true. They must be incorrect. If we operate under the assumption that the culprit killed the victim in the flat, we run into a sure contradiction when attempting to explain how the culprit disappeared from the chained room. Therefore, the killing blow MUST have been dealt outside the apartment. But the victim alone must then enter the flat, so the killing blow CANNOT mean instant death.
The victim, mortally wounded but still mobile, rushed inside in an attempt to save themselves. They chained the door, then perished next to it. This--the body resting against the door--explains why the gap was so small.
Why the incorrect assessment by the police? Fenris' fault. His feat of strength at the door buried the knife deeper into the wound and flung the already dead Beowulf into the position his body was found. Now, as far as the police would be concerned, the wound appears to have been an instant death.