There are the three original killers - Gigi, Boone, and Boone's sister (X) - at the start of the series, who have been plotting this long game to go off this year at the college.
Over the course of the season, two *other* characters have had reason to kill someone, and chose to adopt the Red Devil persona as camoflauge to help get away with their crime.
Which means things get muddled because all the different killings were no longer done as part of the same big plot. Oh, also a really strong possability one (or more) character has a form of split personality.
Original Three: Gigi, Boone, and Grace. Grace was programmed in some way and then sent to her father after 2 years with Gigi, likely with possible check-ins over the years. Her crusader is the perfect foil to make her above suspicion.
Independent Red Devils-come-Latelies: Wes and Pete. Wes killed Mandy Greenwell when he became afraid that she would tell Grace something he didn't want her to know. Pete...depends. If he's insane, he killed Coney. Otherwise, I think Wes and Pete took out the Dickies while the original three kept taking out Kappas.
The major cog in the spanner is that there is crossover between which bodies were found at the mystery house, all of which should (in theory) be victims of the same set of killers.