When a series of murders with obscure motives stain with blood a small kingdom just after the abolition of the capital punishment, the police ends up suspecting the former last executioner to work on his own …

When capital punishment is abolished, a now unemployed state executioner begins a new career as a hit man.
or
When capital punishment is abolished depriving a state executioner of the only legitimate channel for his pathological compulsion to kill, he turns to serial murder.

Granted, the premise, particularly the 2nd one, is similar to the premise of the American cable TV Series “Dexter”. Do you want your protagonist to sublimate his pathology by killing murderers who got away with their crimes — like Dexter — or act it out his homicidal compulsion in random acts of non-kindness?

Whatever, I think this has the makings of a high concept story. Which is to say, you don’t have to explicitly spell out all the details of a protagonist, antagonist, objective goal, stakes. A high concept not has an obvious hook, in a few words it implies all the yada-yada or stimulates the imagination as to the dramatic possibilities. Well, it does in my mind. I am highly interested in the premise.

And I can’t speak for the French market, but I think this would have franchise potential in the U.S as a series of feature films or as a cable series.