I was looking for the same film/movie since a few years. I watch it on tv in 1990 and I only rembember that the killer was blonde hair,
wearing black clothes and he was killing the members of a jury, perhaps they had convicted some kind of relative (his brother?) because at the end of the film
whe he was on a trial and found guilty, it was his parents (an old guy wearing a beret) who take notes about the people in the jury to take revenge on them.
I always thought that the movie was North European: Swedish, Danish, Norwergian, German, Dutch, even British but you give me the clue that I never considerer: it was a French movie.
So, I tried to find out more and finally I found it, the film/movie is: "Une journée bien remplie" directed by Jean-Luis Trintignant in 1973, with Jacques Dufilho as Jean Rousseau (the killer).
Check on IMDB: http://www.imdb.com/title/tt0069443/
Plot/summary: A father plan to kill in the same day the 9 members of the jury who had condemned his son to the death sentence.
And comments from people: "It follows a psycho as he goes on a murdering spree across France, killing Garage attendents, Life Guards, Actors in the most brutal, Penelope Pitstop contrived situations, ie locking the Garage attendent in his paint-oven.
All these murders are unlinked until the final 10 minutes when we discover that they are actually members of a Jury about to condemn his son, the final scence...well, I'll leave that for you.
It's black-humour at it's best, a French genre film at it's best ..."
I think I'm right.
I want to thank you because I was looking for the same film and you helped me with your comments.
Is it that movie/film?
Watch the trailer: http://www.dailymotion.com/video/x4av68_une-journee-bien-remplie-bande-anno_shortfilms