Only Battler seems to notice Maria is different.
Maria is not as evil as she seems.
Shannon has yellow hair instead of brown when she died.
6 people died at the same time.
Eva and the rest of the adults doesn't seem to approve of George's relationship.
Kanon went missing at the 4th episode.

So it makes me feel that Shannon is the villain here in the first arc. She could have killed Jessica and substituted her because both have really big breasts. Motive, possibly because everyone of them disagrees her relationship, but they died accidentally. But the one making Battler seeing hallucinations is another person, which I do not know who and why. Possibly Kanon killed the three servants.

First, after Shannon left George and started her job, the 5 people there may have seen her outside with George, then questioned her, then Shannon gave them supper or drinks and by accident poisoned them.

Soon after, Jessica came in and saw her father dead, she questioned Shannon, then Shannon killed her, or Jessica got asthma and died because she can't take the shock. She can easily carry six of them by carting them with a wheelbarrow, and also an accomplice, a servant that helped her do the job.

Wondering what will George react if she murdered six people, Shannon then proceeds to switch clothes with Jessica and tore all of the corpses face so that they will not recognize their face and also prevents them wondering why one of them has her face ripped.

Six hours later, Natsuhi went to have a talk with Kinzo, between Kinzo's comments and Natsuhi leaving the room. Kinzo may suddenly have a heart attack and died instantly. Natsuhi panicked because if she were to be found with a dead old man, she will be instantly be accused. She proceeds to throw his corpse outside the window, then acted as though nothing happened when she met Eva. She then finds an opportunity to dispose of the corpse. So everyone was wondering at all of the deaths.

Natsuhi has become increasingly freaked out (she's starting to have a shotgun) Then after Eva angered Natsuhi, she and her husband went into their room. Natsuhi may have had an alliance with Genji, having Genji poisoned Eva and Hideyoshi, then make it mysteriou

Then after Kanon got stabbed, the fake-Jessica took care of him and told him that she was Shannon, and told him the real Jessica is dead. She can easily lie and said it was all their fault.

When they saw Kinzo's burned corpse, the servants can easily suspect Natsuhi since they worked there. So Natsuhi may have allied with Kanon and finds an excuse to rid of the three people, by putting the can of sardines together with the supposed witch's letter. She have Kanon killing them all. Kanon made the same method that Shannon did. Natsuhi can have Genji removed as well to get rid of evidence.

-Natsuhi is the last to die, while all the others are locked. In the anime, we can hear only one gun shot. If it's not a third party, the bullet can have bounced off something. Straight into her head, hrm.
-The previous three died while only Maria was around and everyone else was locked together. Either Maria somehow did it (on three adults... ?), or someone else did it, or one of the three did it and killed themselves.
-What do we have so far on the island ? People who may want the money, people who may believe in the witch and who would actually kill people to "revive her", people who could simply want the others to believe in Beatrice, and maybe accidents. A big possibility is that not all the killings were done by the same people, even - I mean that without even anyone being conspiring together.
-As an example of the above, if Natsuhi did some of the killing, the last three (before her) couldn't be killed by her, and seeing their death, she could freak out, which could lead to her believing Beatrice really was around, and do something that could result in her bullet bouncing back and kill her.
-Another example, for the locked room with George's parents, the mother could kill the father and then someone else could kill her - she could even kill herself. But you'd need a pretty big motive for that.
-Whether one of the dead bodies (any of them) was swapped (Jessica/Shannon or anyone else) doesn't help much in saying how the murders happened.
-Genji is the most likely to be able to have the physical force to do some of the stuff that were done - carrying bodies (including Kinzo) and whatnot. He could also be capable of doing the last three murders before Natsuhi, and then killing himself, if we follow the possibility that he wanted Beatrice to come back, or if he wanted people to believe in her. For some reason.
-At the very least, there's someone who does want people to believe in Beatrice, or who believe in her themselves. because of the drawings and scribbles and all.

Kanon went missing at the 4th episode.

? She dies. The doctor goes to try to save her, Jessica comes with him, then when they come back, Jessica falls down and cries - looks like Kanon simply died from the wound.

Kanon clearly believed in Beatrice and the attempt at reviving her, given his/her speach and suicide attempt just before getting knife'd.