The killer is most likely Melanic Dorkess (spray tan girl). First, way in the beginning of the show, Ryan Murphy took to twitter to say that we've already seen who the killer is, and I doubt that the remaining people in Kappa are the killers. Looking back at Melanie's death and assuming Chanel was responsible for burning her, it's clear that maybe Melanie didn't actually die and came back to get revenge on Chanel for trying to kill her AND stealing her position. This could explain why Chanel hasn't gotten attacked yet, as Melanie wants to kill everyone off before doing so to Chanel. Remember how Chanel always says a backstabber deserves to die, who else would believe this? Melanie!!! Let's also remember that Melanie mention "dancing with the devil," heard that phrase before? It's the same thing the red devil texted Chanel #2 before killing her!! And lastly, Melanie saw the red devil walk in, so maybe the red devil (Boone) who could also be her brother, helped her live and fake her own death? It makes sense why the most forgettable character would be the RD.
If she's not the RD then my bet's on Chanel #4, i'm not buying her whole "Meningitis" act. lmao