Hey guys, so this isn't so much a theory as a list of all the character's who can't be the Red Devil, to help us all keep up.
Not the Red Devil:
She confronts Gigi about being the killer in private, isn't the bathtub baby.
Gets kidnapped by Boone.
3. Dean Munsch
4. Chanel #3
Is very surprised and afraid to see Boone alive.
Hadn't met Gigi before she came into his car in episode 2.
Very surprised to see Boone's ghost and would never kill his frat bros.
Wouldn't murder Shondell, was attacked
I know some of these people are too old to be the Red Devil, but maybe just incase there's one more person working with them who's older (though I don't think there is) I wrote them down.
Let me know if I've forgotten anyone