xSinghx: Hints:
Italian Neo-realist filmmaker
Joined the Communist party to help organize anti-fascists.
Made only 5 non-documentary features which were released from the late 50s to late 70s.
Adam Curtis uses the theme of this movie in one of his documentaries.
I was bored and saw that this thread had stalled, so I did some minimal research based on your hints and it sounds like this movie is
Ogro, aka
Operación Ogro (1979) by Gillo Pontecorvo.
EDIT: And
this trailer including the screenshots confirms it, so I'll just go ahead and post the next one, if that's alright. So here goes.
HINT: Sadly, this movie is not that well known, but the actor is easily recognizable. Also, it's from the 90's and there are many movies with the same title as this one, especially B movies.