Tenebre/Tenebrae (1982)

The successful American writer from Rhode Island Peter Neal travels from New York to Rome to promote his new best-seller Tenebre. He is received by his agent Bullmer that schedules an interview in a talk show. As soon as Peter arrives, there is the murder of a shoplifter and Detective Germani is assigned to the case. He meets Peter and tells that the killer was inspired by his novel to commit the crime. Peter receives a letter from the murderer and sooner two lesbians are murdered. The killer writes that perverts must be eliminated and Peter suspects of the host of his show. However, when you eliminate the impossible, whatever remains might be the truth.

IMDb: https://www.imdb.com/title/tt0084777