Literally anyone can be the killer, but after reading some of the things pointed out Grace could be the Red Devil.
Ryan said if you re-watch Ep 1 It's so obvious who the killer is, and It's really in your face that somethings up with Grace.
"I Need to Get Chanel and her minions out so I Can do it." (Change Kappa to the way she thought it was) What's a way to get them out? Kill Them.
The fact that she hasn't been attacked by the RD is suspicious too.
