In the early 2000s, I saw a low budget US thriller that was quite decent. On DVD or as stream, in color. I think the movie was also produced around that time.
What I can remember from the plot: The movie took place in a city, maybe New York. By going berserk, a detective barely survived a brutal shoot-out with many gangsters in an old building. Later the audience learns that a mysterious killer/hitman, kind of a psycho, had his fingers in the pie, and that he was impressed by the detective’s will to survive. Therefore the killer decided that the detective should become his successor.
He used hidden cameras to stalk the detective, killed his partner, etc.
Towards the end of the movie, the detective discovered something horrible in his apartment: hidden behind a curtain or large poster, attached to the wall: his pregnant girlfriend/wife, killed by the psycho hitman.