The Black Room (1982)
A serially unfaithful businessman rents a spare room from two siblings to use for his sexual conquests, unaware that they're serial killers who murder every woman he brings there.
A room gets put up for rent following the mysterious murder of its previous tenant. When an apartment-hunting man sees the deal the brother and sister landlords are offering, he immediately puts down the first month's rent and starts spending his days there, even though he has a home to spend his nights at with his wife. When he comes home, he tells his wife about the fantasies he has of having sex with various women each day in his new apartment. His wife goes along with it for a while, but when she discovers that her husband's stories are much more than just fantasies, she starts to do some investigating.