Moll was almost certainly the killer. She was having dreams of attacking the victims (the people in the masks are always revealed to be her), her neck hair grew out (beastly motif), her nose bled because she was feeling sexual arousal from the deeds being shown to her, she loved Pascal’s “smell” and not the police officer’s smell, she felt somewhat guilty about what she did and tried to get people to say she was a nice person to no avail (the girl she stabbed telling her to get out, the funeral where she screams like a beast at the people harassing her, and the people not wanting her there), going to the officer to get Pascal in trouble because she didn’t want her deeds known, the Mother being extremely oppressively strict (her mother obviously knows and has been hiding it from the town), and the kicker is the scene after Pascal “confesses” after she forces him to because she “knows” without evidence . . . she proceeds to make him into her next victim and gets out uninjured like a Beast. Pascal’s heartfelt cries of being like her being choked out of him as she strangles him just like she did her other victims.
Every motif points to her being the killer; it’s just not outright said. Also, the story conveniently has Pascal set free once the Portuguese man is arrested. If it was Pascal, why were Pascal’s prints not all over as hard evidence? Because it wasn’t him; she was using him as an alibi, the police stupidly told her of her own deeds making her feel horny because she was getting away with it, and it ends with her making Pascal into her next victim. Also note: she wanted to leave the island, not Pascal. She was the killer and her mother keeps covering for her.
The event that convinced me though was the grave scene, where she put dirt all over herself and covered her mouth with it. She was in the grave of one of the victims embracing the smell of death and sleeping in the grave the victim was buried in. She gets out the next day in a very beastly image before cleaning herself up. She was the killer. There was no reason for her to do that, if she was feeling guilty and that wasn’t some random grave. The detective from out of the island had shown that picture to her, she got sexually aroused and had a nosebleed, and after she dumps away her guilt; she sleeps in the dead girl’s grave and enjoys the smell before rising as a Beast to meet back with Pascal. Pascal becomes her next victim at the end. Her mother is just going to keep covering for her deeds. Moll clearly hates the town and hates the people who have a better life than her; Pascal is given no motive whatsoever for his supposed deeds. The story was heavily hinting Moll was the actual killer the whole time.