Okay, so this is a theory a friend and i has just figured out.
We are picking up after The Chanels (Chanel #1, Chanel #3, Chanel #5 & Chanel #6 aka Hester) are attacked in The Mall on Black Friday, in the episode 'Black Friday'.
And from earlier on we know that there is only one killer left, since Boone was stabbed by the still alive killer, and Gigi aka the mastermind got her head cut off and served to Thanksgiving, in the episode 'Thanksgiving'. This means we only have one killer left, and the information we have gotten earlier says that the last killer must be twin in the bathtub, since the brother to the sister is dead (Boone).
So The Chanels (Chanel #1, Chanel #3, Chanel #5 & Chanel #6 aka Hester) are being attack in the mall, and we see Chanel #3, Chanel #5 & Chanel #6 aka Hester take off, but Chanel #1 stays to tell the killer that she knows it is Dean Munsch. But when she got shot, Denise Hempfill shows up whilst the killer is standing in front of them.
^to sum up: The Chanels (Chanel #1, Chanel #3, Chanel #5 & Chanel #6 aka Hester) and Denise Hempfill can not be the killer, since they have faced the last standing killer, meaning that there is only one left.
"What about Dean Munsch" you may ask, and i can tell you, she is not the killer. Since in the episode 'Mommy dearest' she is attacked by 2x red devils and a third masked killer wearing a Judge Scalia mask. So we can assume that one of the red devils is Boone and the Judge Scalia killer is Gigi, because Boone revealed it over a phonecall later in the same episode. Which leaves us with the last unknown killer.
^^ summing up: ^to sum up: The Chanels (Chanel #1, Chanel #3, Chanel #5 & Chanel #6 aka Hester) and Denise Hempfill can not be the killer, since they have faced the last standing killer, meaning that there is only one killer left. + Dean munsch was attacked by all 3 killers when they were alive, so she can not be the killer, since she has faced all of them at once.
So the interesting one, Zayday. I don't have anything exactly on her, except the fact that she was held a hostage by the red devil (in this case Boone). But in the trailer for the next 2 episodes, it is shown that she is the one taking the mask off the killer = it can not be her.
Which leaves us with one girl left. Grace. And it all adds up now. She has never faced the last standing killer. Earlier in the season, she was like "omg peace, no violence, happiness, no killing" and etc. but in this episode 'Black Friday', the moment The Chanels says, that they think it is dean munsch, she completly agrees and brings the suggestion of killing Dean Munsch. Which meant that it dragged the attention of her as a killer.
She is the only girl with relevans towards the storyline left in the show. - more ment like a comment, for people saying it is Melanie Dorkus or Chanel #2, it would not make sense, because they dont have any relevans for the storyline, in the way like grace does.
Now you may ask, "why do you think Grace is the killer, why would she do such?": In my honest opinion i dont know. Maybe it is because she knew about 'her mother' before her father told her, or maybe she wanted revenge on kappa for killing her real mom, because they all rather would party than drive her to hospital after giving birth.
^^^^ Grace is the killer.