I've read a theory , and I think it's the most appropriate
the culpirit(s) is not one of the 18, but an outsider
since beato stated in red on the end of the four game that there's no one on the island and she will kill battler .. that's don't exclude that on that time the culpirit was on the harport of kuwadorian on a boat ( he's on the sea , not on the island ) and he can join the island and kill battler
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