shadow on Karren's face at the very end
spoilers
If you notice after she "sees" the demons she closes her eyes and they seem to be gone. You see a large shadow come from the bottom of her face to the middle of her nose. She then has a more peaceful look on her face before the movie goes to credits. Theres seeral thing this could be, If you take the religous view it could possibly be jesus or something similiar. I tend to think it's her boyfriend come to rescue her.
I firmly subscribe to the muffins having a lsd type substance in them and noticed this on a second viewing. karen only started freaking out after she ate them. And the maintenance guy mentioned eating them before he saw a "demon" Also several characters seem to pop in and out of this alterted maniacle paranoid state. Which would mean that they are under some sort of influence that alters their personality, which could be the drug losing it's affect or it doesn't have 100% affect on their personality all the time or may kick in sporadicaly as its wearing off. Even the terrifying cult leader lasy seemed to come out of her fugue before she died and seemed to be more like a normal person. I think she asked Karen to kill her first because suicide is supposed to be a sin and then when she didn't, since she was already going to die and realised all the horrible things she did she took the pill.
I don't think there was anything supernatural in his movie and it is condemnation of religious extremest who try to force their views on people or even murder "Non believers."