[izmosmolnar]

Spoiler for definite truth:

Yeah them seagulls are certainly suspicious, since they are around the island in episode 3, despite that Eva isn't noticing them for whatever reason.
However the likelihood, that there must be a reason why Kuwasama is so obsessed with Mackerels seems more suspicious. Additionally with the exception of Eva-Beatrice's "red web nightmare" Beato never said, the murders aren't commited by animals.
So connect the dots together, and there you have the final truth: The killers are Mutant Mackerels. The motive is currently unknown of course, but I would expect the answer arks to answer trivial questions like that.

Anyway, back on being SRS. I would also like to see one of the answer arks, to elaborate a bit more about the story with Kinzo and Beatrice, plus the contract they made, how did he managed to make his business to prosper, why is a Beatrice living in the second Mansion etc.