One of the most striking motifs of ‘A Madman’s Diary’ is the anthropophagism that is first hinted at and then becomes a central obsession of the diarist. One of the first questions this raises is: “For what is eating people a metaphor in this text?”, assuming of course that it is to be read metaphorically. Given that the text 
The history of literary anthropophagism is long, complex, and beyond my competence to sketch out in any detail. Whereas cannibalism is frequently portrayed as taboo and a categorical transgression the evil or repugnance of which does not need to be argued, there are also notable exceptions. Michel de Montaigne’s Of Cannibals presents a culturally relativistic picture of anthropophagism, observing that
I find (as far as I have been informed) there is nothing in that nation[1] that is either barbarous or savage, unless men call that barbarism which is not common to them. As indeed we have no other aim of truth and reason than the example and idea of the opinions and customs of the country we live in.
Anthropophagism also raises the theme of corporeality and, for Lu Xun, the notion that one takes on the characteristics of that which one consumes: ‘But just because I am brave they are the more eager to eat me, in order to acquire some of my courage’.
In ‘A Madman’s Diary’, anthropophagism is allied with secrecy: the diarist does not know simply by looking at someone whether they are a flesh-eater or not. This exchange from the 8th entry captures well the diarist’s suspicion and attempt to ‘out’ the cannibals:
Suddenly someone came in. He was only about twenty years old and I did not see his features very clearly. His face was wreathed in smiles, but when he nodded to me his smile did not seem genuine. I asked him “Is it right to eat human beings?”
Still smiling, he replied, “When there is no famine how can one eat human beings?”
I realized at once, he was one of them; but still I summoned up courage to repeat my question:
“Is it right?”
“What makes you ask such a thing? You really are . . fond of a joke. . . . It is very fine today.”
“It is fine, and the moon is very bright. But I want to ask you: Is it right?”
He looked disconcerted, and muttered: “No….”
“No? Then why do they still do it?”
“What are you talking about?”
“What am I talking about? They are eating men now in Wolf Cub Village, and you can see it written all over the books, in fresh red ink.”
His expression changed, and he grew ghastly pale. “It may be so,” he said, staring at me. “It has always been like that. . . .”
“Is it right because it has always been like that?”
“I refuse to discuss these things with you. Anyway, you shouldn’t talk about it. Whoever talks about it is in the wrong!”
These themes strongly resonate with a number of 1950s sci-fi films, notably Invaders from Mars (1953) and Invasion of the Bodysnatchers (1956) (trailers below).
https://www.youtube.com/watch?v=WFnSxeDfENk
https://www.youtube.com/watch?v=Ury5b-qtI1Y
Finally for this post, anthropophagism raises the question of the human/animal distinction, also thematised in Kafka’s ‘A Report to an Academy’, studied in this course.
There is a great research essay (or two) to be written on anthropophagism in modernist literature, and also on the different ways of thinking the human/animal distinction in the period.
[1] Montaigne is referring, it appears, to Brazil.