What I remember about this movie is: It was made more than 20-30 years ago.
I think the guy/killer lived in the whole house at first but for some reason the house needed to be sold and he wanted to stay, so he converted a lower floor bathroom to a hidden room by walling over the doorway. I think he wallpapered it a reddish color. He put a small picture on that wall where the door was and right under the picture he put a tiny hole so he could peep at the new owners. He stayed in the room and to get out, there was a trap door on the floor and he would come and go from under the house (I remember lattice hiding the way under the house).
Not sure who all the victims were but I think he killed or tried to kill a neighbor. I recall that the new owners of the house thought it strange that there was no bathroom (as it appeared) on the first floor. Anybody know what movie this is?