Maybe some people think that the murderer is the boyfriend at the end, based on the nosebleed after the murder; some people think that the murderer is the boy who was stimulated when he was a child, and it seems that he has been laying the groundwork from the very beginning; but I think that the murderer is exactly the one who is revealed at the end of the movie, and it is one of the girls who took off the mask.
First of all, as recalled at the beginning of the movie, the little girl was the uninvited "ugly duckling" at the ball, but she framed the boy who had invited her, and in addition to the little boy being hurt at the ball, she herself was also very hurt. In the second half of the movie, it is also mentioned that the grown-up girl confessed to her friends that she was fat and had low self-esteem when she was a child, and we can see that she was full of jealousy and hatred towards her beautiful friends. Secondly, she repeatedly draws attention towards the boyfriend with alcoholism to make it look like he's the murderer, partly because of her jealousy and partly because maybe she's just trying to frame her. Also, it's true that she wasn't present at any of the previous killings, so it's quite possible that she was the killer.
Fourth, if the murderer was someone else, why kill her boyfriend and her maid? It doesn't make sense right. And since her boyfriend cheated on her and her maid humiliated her, her being the murderer would be a good explanation.
Fifth, I think in the end it was everyone who died, the girl died in her boyfriend's arms, and the boyfriend had a nosebleed maybe because he died of poisoning from drinking and eating her family's food.
Sixth, this was a rather surprising ending, and if it had been revealed at the beginning there would have been little suspense. Because I have been following the opening credits and decided that the murderer is the little boy, and continued to watch just to find out how the murderer will be caught in the end. But the ending really took me back a bit.
Personally, I don't think that the man who was dating frequently at the beginning of the movie was a grown-up boy, but just an obstacle set up by the director. Because based on the judgment of the police to make facial photos later, it is not a face at all, it is impossible to change a face.
Perhaps the director just wants us to make a blind guess, who could be the murderer.