End of the Game (1975)

While investigating a high-profile murder case, a savvy but unorthodox veteran police inspector has to cope with a bad conscience, bad health, an overzealous partner, a timid superior and interference from political interests.

This is an existential whodunit, but a good one, and like any good whodunit, ends with a very surprising conclusion, which will be spoiled for you if you read much of anything at all about the movie.