So, I've been totally lazy with my reading of Redaction, but now that the 1994 narrative ended and I still have no idea how it connects to the Rokenjima one, I feel stupid for letting everything go like that, so it's high time I started theorizing a bit.
Spoiler for Culprit Ideas:
It has been stated repeatedly that Andromalius is an assasin. Which might mean that the one killing everyone doesn't necessarily have any sort of grudge, but is doing it for someone else. And also, if we go by the 1986 narrative, the victims were given a chance for surrender which they didn't take....
I am pretty suspicious of Jeroboam, to be honest. He seems to be very aggravated by the others' 'Disneyland' view of Rokenjima, yet he still, albeit reluctantly, goes along. Why? He isn't tolerant of this sacrilege of the dead, he even criticizes others for it, and he doesn't seem to really enjoy their company either, so he must have had another motive for tagging along. According to Renall, the name Jeroboam hints to something, at least symbolically, so perhaps 'his people are many' supports the assasin theory.
Another reason why he is suspicious, is the fact that we don't even know his name. According to Chessboard Logic, the author wouldn't want us to know his name, because it would evoke some sort of reaction, which may mean that he is someone that we already know. Now, he certainly isn't one of the 1986 people because his age doesn't fit and he sort of isn't even Japanese... So his name may link to one first mentioned in this forgery. What if his real name is Andy or something similar that clearly points to Andromalius?
I'm also suspecting Cass (even though her final moments made my heart crack). She is the one who proposes the trip in the first place, and she mysteriously finds the means to get there while she ditches Karl. Plus she is the only one who had ties with everyone else before they got to know each other.
P.S: I can't stress enough how much I love Andromalius' sprite. First of all, it is very orignal, and he is adorable, but more importantly, his trollfaces look really similar to Ryukishi's art!