This 1949 documentary by Georges Franju is probably the most beautiful film ever made about slaughterhouses. Like many things I post here, this is not for the sensitive or the squeamish. (I really mean it this time.) But if you are open to an admittedly upsetting experience, you will find much to ponder in this great film.
(Annoyingly, this playlist runs out of order. Click the segments here manually and you'll see it the right way.)