[TOMT] [made for tv? movie] [Horror/sci-fi/noir] [2000’s]

I watched a movie on tv as a kid that starred a detective who was solving a series of murders.

It turned out to be some kind of monster/alien that would appear as a person the victim trusted. In the finale the detective confronts the monster who appears as his elderly neighbor. She keeps approaching him after he tells her to stop and then he shoots her (and it’s the monster). He says he is surprised he finally trusted someone.

It is very noir and maybe black and white.