John Phillip Law in "L’occhio Dietro La Parete" aka "Eyes Behind The Wall" (1977)

Ivano is a wheelchair-bound man with some strange perversions. He gets his  sexual kicks by spying on Arturo (John Phillip Law), their tenant in the apartment next-door and forcing his sexy accomplice Olga to watch as well. Ivano persuades Olga to seduce Arturo and to have sex him as he watches. Unfortunately for all involved, the sex game starts to get very deadly. IMDB: