I remember this horror movie from late 70s early 80s. I think it was based in the woods in a house close to a river or the river may actually run under part of the house.

Someone kills anyone that comes near the house.

I remember close to the end a teenager girl finds what I believe was a boy locked in a dark room with the mirrors covered.  She tries to help the boy by taking the cover off the mirror so he can see himself (I don’t think she even saw his face at this point).  When he turns to the mirror she screams and I think runs out but someone catches her and kills her (not sure if it was the deformed boy) I think the killer might have been a woman like maybe the mother.

I saw this scary movie in the late 70s early 80s.  I want to say it was good vs evil.  I remember something about a large Catholic church and something about the devil.

The part I remember best is when a man rigs a way to kill himself.  In his office he sets up a shotgun by tying a rope to the trigger of the gun then the other end to the office door.  He placed the gun so it aimed right where his head would be while sitting in the chair.  He sits in the chair and when someone opens the door the gun goes off and blows his brains all over the wall.

This movie has driven me nuts for years, no-one remembers it and I want to see it again.