Film: Dark Corners [SPOILERS]

This movie has left me very confused. As i understand it, this girl Karen (the one in the dark reality) is spending eternity in hell, being tortured for her sins on earth. The other girls life, Susan's, is experience by Karen through dreams so as to heighten the torture ("candy before the medicine makes it taste all the worse"). So if this is Karens torture, why does Susan also experience Karen's reality through dreams aswell? Then there is the serial killer, who i guess is Karen's tormentor. But right at the end, we see Karen's face morph into that of the serial killer. I guess because she commited the sins willingly, she damned herself to hell thereby shes her own tormenter? But then, shes the therapist. And thats where i get lost. Who is this therapist? Hes the killer and he's Karen, and im not following. What did you think of the movie? Who is this damn therapist?