Eyes Behind the Wall (1977)

The movie starts with a scene of a young man raping and murdering a woman on a train. Later he rents a room from an elderly wheelchair-bound voyeur and his much younger wife. The elderly man enjoys watching the younger man through the a hole in the wall where he has set up a periscope-like device. The first thing he sees, for instance, is the younger man get forcibly raped by a big black guy he has picked up at a cheesy disco. Then he sends his wife over to seduce the young man, and its eventually revealed the true nature of the couple's relationship and how he came to be in the wheelchair.