I think Gigi, Boone, and Pete or maybe Hester are the red devils.
First of all it's pretty obvious that Gigi is one of them since she is the hag of Shady Lane aka the women that kept the baby(ies?) of the girl that died in the tub. She has been planning the revenge plan and has been seen talking with the red devils. I don't really think she's actually killing people. It just seems she's there to trick people and making conclusions of stuff, ecspecially when it comes to Grace and her father. But yeah I think why she is doing this is to get revenge on Dean. Because she and the Dean has a really rough relationship. But yeah, she's eventually gonna go, since I'm pretty sure Boone and the other red devil are planning to kill her. I think it's because Boone isn't really just targeting Dean, but instead he is targeting Chanel or someone else that had to do with the bath tub targedy and that he think with having Gigi his own plan will take longer and is hard to do with her around. So, Boone is definetely one of the red devils. Wait, but maybe he is also the baby(ies) in the bath tub. Like in previous episodes they told us that the baby that the girl in the bath tub had was a girl. But, plot twist at the mental hospital it showed that she holded two, yes two babies and one wore pink and one wore blue which means they're a boy and a girl. Maybe, just maybe Boone was the boy baby and that's why he has revenge on Kappa Kappa Tau, ecspecially at Grace's mom that was the friend of the girl that die in the bathtub (Boone's mom?) who didn't help her. And for the last devil, I'm not really sure who it is. It could be either Pete, Hester or maybe both! Since the other baby is a girl, it can be Hester. Since she is the most suspicious and weird in Kappa and that she really hates Chanel. But then again, if it was a boy it would definitely be Pete. Since he has the costume, how he has outtakes of miscivious faces throughout the episodes like when Grace found out he had the red devil costume and he looked at the camera with a smirk and almost smiled. Very suspicous, he coulod also be the baby. But, I dunno. There still more parts to come but yeah. This so far is what I think. I also believe some of them did killed but isn't the red devil. As like how suspicous Dean is I don't really think she's the killer. She did killed her ex husband. But she isn't the killer. She hates sororites but cares about herself and her reputation more then that.
That's my theory. But, I'm reallty not for sure of it snce there's 4 episodes to come. So it can possibly change.