The description for the Massacre achievement for Monaco on Steam says
Kill everyone on a single mission
On some missions, there are civilians as well as guards that you can kill. Now I know you can just go to a mission that doesn't have any civilians and then just kill the guards, but that still leaves the question; is it mandatory to kill civilians as well as guards in order to get the Massacre achievement?
Best Answer
According to the developer, yes you must kill civilians as well as guards in order to get the achievement. The easiest level to do this is Hijack at the Hairpin.