The problem is less who, but why and to what extent, and that's something Eva presumably didn't know. Battler's involvement, for example, is portrayed in two distinctly different ways: In the ep7 Tea Party he seems very much against any criminal activity, but in Bern's ep8 purple game he's outright helping his parents commit murder.
I think there are a few common elements that seem to indicate something is up:
- Yasu was planning a fake murder of some sort, but what she really wanted was for the epitaph to be solved so someone could stop her.
- The epitaph was very likely solved.
- At some point, people other than the adults die or appear to be dead; Eva seems to know or believes she knows how the other adults died assuming we take the ep7 TP as similar to her diary (although it can't be a fully accurate depiction), but she didn't directly witness any other deaths as far as we're told so far in the VN and manga.
- Eva believes that Kyrie and Rudolf were responsible, and they do not deny it, but neither are their motives as clear as Eva might insinuate she thinks they are. Kyrie's final speech about Ange in particular is clearly manipulative and carefully designed.
- Eva appears not to know the extent of Battler's involvement, and may not have had any contact with him whatsoever. He effectively seems to just disappear, but we know that he survived somehow.
The thing that sticks out to me is the notion of somebody faking their death. It happens constantly:
- Shannon fakes her death in ep1, then Kanon does the same.
- Arguable in ep2, but Kanon didn't "really" die and appears later according to Gohda.
- Yasu is supposedly faking death in ep3, and some interpretations allow this to be true for other characters (briefly) later.
- The circumstances of Kumasawa and Gohda's deaths in ep4 have the appearance of a fake hanging, although they turn out to be real murders when Battler investigates further.
- The First Twilight of ep5 is clearly faked.
- Battler fakes his death in ep6 along with the others. Additionally, Erika kills people who are faking death to "confirm" it.
- Eva isn't actually dead in the ep7 TP when shot and essentially "returns from the dead" later.
- The ep8 purple game revolves around Battler lying about his parents' death.
This leads me to believe that
someone was faking their death, but I'm not sure how this ties into actual events. There's an obvious impetus for someone to do so, because Yasu apparently intended the murder game to be fake. The bomb was the fallback according to the manga, so it reads like she wasn't going to directly murder anyone under any circumstances and just let the bomb go off if nobody stopped her. The question is how this squares up with the narrative of the adults fighting over the gold (because it seems like a fake murder would be irrelevant if the adults started the chaos by shooting one another) and with what Yasu was doing. The other unanswered question is who solved the epitaph. We're told the parents can solve it together, and we're also told that Battler has the ability to solve it. Ep7 implies the adults did it, but ep1-4 implied that the adults were usually not inclined to do it until the fake murders started because they simply didn't take Beatrice's letter seriously.
So I wonder if either Battler was the one who solved the epitaph (and informed all the adults) or if Battler participated in faking his death as part of Yasu's "murders" and this spurred the adults to solve it and then tensions ran high because they didn't know the deaths weren't real. Kyrie snapping and going on a killing spree would make an awful lot more sense if Rudolf had just told her that Battler was her son
and Battler was apparently just murdered, for example. In ep6, when Rudolf believes Battler is dead, he has a breakdown about it too. If the adults had some reason to suspect the other adults then
anybody's children turning up "dead" would put them on edge. Although then we run up against the problem of "Beatrice's" confession to them in the gold room. Given that she's playing up the witch role you'd think she'd mention something about how everyone who "died" was "revived" if that was part of the script. But maybe she didn't get that far before the shooting started.
The missing point of view in all of these events is Battler's. The VN didn't touch on his direct recollection of events very clearly (unless, as has been suggested, ep4 was sort of his perspective on events), and the manga may or may not. It's especially important in the instance where Battler saw something really important that Eva didn't see, or knew about something Eva didn't know (like a murder game). It's obvious why Ange might suspect he was involved since popular theories at the time often included him as a culprit in theories about his family, but he's really the only person who would know for sure as Eva seemingly didn't.