Okay a bit unlike everyone else, I don't really think Rosa had anything to do with the first murders.
First off there is of course the issue of Rosa going straight for the envelope in Maria's bag, which ended up containing the key to the chapel. One could assume easily that she did knew the key was really there (a possible sign of guilt) but again it seems to hang on the presentation of the narrative. The narrative states that her first thought after seeing the chapel locked and the magic circle was to think of the letter Maria received, and to an extent I have to believe this because the narrative is supposed to be objective to a degree, and it is the closest to truthful exposition as we can get. I would believe her first thought was a magic circle<->Beatrice connection, and by extention think of the things connected to the so called witch. The letter is their only current link they have and when you consider the ominous words of "the time will come to open the letter?" that Beatrice says, and would naturally go for the letters. The missing family members is suspiscious enough considering the warning the witch gives. It might be that the key just so happens to be there for her to find.
This again I am assuming the objectivity of presentation.
As for finding the key, the problem is how the "witch" got possession of it. Considering that Rosa didn't seem to know where it was we can assume that the none residents might have not known either, which reduces some of the suspects the the residents + servants. When you think about it, from the anti-magic POV, since this "witch" did give Maria the letter much earlier then the key couldn't have been inside at that time or else no one could have used to Chapel.
The biggest clue for me is that since the witch's presentation happened inside the chapel, someone had possession of the key that very same night to let them in. This means that if none of the adults who died had possession of the key at the point of death, and that the key ended up in Maria's bag, the only one who could have brought it out was someone who knew this witch gave Maria the letter, if not the very same person. If this is a single person's doing, the person of Beatrice (wether real or not) would be the only one who could likely do it.
That being said, the key must have been dropped into the letter AFTER the murders were committed, which calls into question access to the room. While it's easy to assume a guilty servant could do it, I'm not to sure about accounting for everyone's presence at that time. That said, since there's no reason for them to remain in the chapel after their discussion, the killer should have already been there in some form to kill them during before their discussion ends.
I doubt Battler and Maria could do it, George could still have been with Shannon at that time and not be in the chapel, Nanjo would not likely to privy to the key's location, Gohda apparently isn't trusted to know where it is and Kinzo has all but left it to the roullete's decision. This to me limits the suspects to Kumasawa, Genji, Kanon and Jessica. Upon further consideration, one expects that Genji will follow and believe his master to the letter, which reduces the likelihood he'd have an active role in the murders. Kanon is himself torn between the Golden Land and his regrets, however he still ardently believes in the Witch, which reduces the likelihood he'd get in the way of the ritual.
This leaves me with Kumasawa and Jessica...
When you think about it, it's even possible they did it together. Both are pretty familiar with the witch's lore, are residents who know the island well and are... well women who can impersonate the witch if ever possible. When you consider it, both of them weren't paticularly visible during October 4, but that is a very loose assumption.
A compounding factor for me is the contents of the second letter in Beatrice's Room. It is honestly far to convenient, plus the letter itself is far too out of character for the witch. The letter was written intently to make the reader angry. Furthermore, the letter was supposedly written in the prediction that one of the children would barge in and find it, that and the assumption that the other 3 children aside from Jessica wouldn't know of the intricate room details. Jessica knew that there was a room specifically for Beatrice and knew where it was, knowledge that might not be known to the other 3.... which makes possible that the letter was intently written for her. If the letter is supposed to be written for one of them to find, but in the end only Jessica would know about the room, to me it casts some suspicion that the letter was written as a cover for Jessica to make a further case for her uncontrollable "rage", rage that would reduce suspicion on her...
Don't even get me started on the hallway convo between Kanon and Beatrice. If we consider that everyone accounted for is still in the chapel, the only people who could fake Beatrice (again assuming an anti-magic POV) is Kumasawa and Jessica...