MGM's Peace on Earth
This was the darkest cartoon I have ever seen. It was created in 1939 and trying to promote peace but literally showed the last two men on earth killing each other. Mind you, this was a story being told by a grandfather rodent to his little grandchildren as a bedtime story. He leaves at the end. I assume to go to "Cafe de Nuts" (which is literally in the new "Peaceville" village) to enjoy his evening. It was extraordinarily humorous without any riffing. I am 100% certain this would become an instant Rifftrax classic.
I know the one you mean. I think it was a remake...