The movies might belong to years between 70’s to 90’s. It’s a crime movie. I watched it when I was a kid ( I know it was not rated for my age but I watched & liked it.) so I am not really sure if it was French or British but based on my blurry memory of police uniforms and hats, I think it was French.
Police force finds a large sealed dinning room where several dead bodies lied around a dinner table. Based on a video footage taken by the killer himself which is found by police in another room around the crime scene, killer had poisoned their food and killed them all and left their bodies & video footage there, intentionally.
During the movie, we understand that killer was taking revenge and the vicitms were familiars (They were invited to dinner by killer.) who have done something bad to the killer. ( I cannot remember what they had done.) and police detective finds that he is still searching for other guys to kill them in order to complete his revenge.
Lastly, killers goes to a live music concert and finishes off his last victim who was one of the orchestra’s musicians. Police arrests him, but he commits a suicide by taking a poisonous pill.