My theory on who the killer is. My guess is that there are the real killers (the ones Boone and Gigi are plotting with) and then other characters are also killing and blaming the murders on the Red Devil. So every character is the killer in some sort of way. Which would kinda suck, but I wouldn't be surprised if it happens
Tbh. I mean it'd be the perfect opportunity to kill someone if you wanted, as you could just pin it on the red devil (so long as you don't get caught)