from memory this gritty and brilliant french movie(also set there,maybe Paris)went something like this;

a corrupt cop made extra money by violence and blackmail whilst on duty.
however,one victim-an old man-comes forward and is led by the cops' unsuspecting colleagues through the station in order to possibly identify the guilty man in person.

as it was a process of elimination, our cop was called back from a case he was on(or corruption), to the station to be screened.

knowing he certainly would be recognised, he threw ACID in his own face before returning- thus evading recognition when in hospital.

after a rapid self-discharge from hospital, he continues his trail of destruction and (i think) dies in a violent shoot-out at the end!!

any ideas anyone?..pleease!!!