Many of the characters who are typically classified as monsters are driven by a single negative urge: their insatiable appetites. Characters like Shelob and Sauron (who, in my mind, are two of the many of the parallel figures in Tolkien’s legendarium) are generally considered irredeemably evil. Their hunger is both literal and figurative. By time she encounters Sam and Frodo, Shelob has “bloated and grown fat with endless brooding on her feasts” and desires for herself “a glut of life, alone, swollen till the mountains could no longer hold her up and the darkness could not contain her” (Two Towers, “Shelob’s Lair”). Faramir describes Sauron as the “destroyer who would devour all” (Two Towers, “The Window on the West”) and even Gollum claims that Sauron will “eat us all, if He gets [the Ring], eat all the world” (Two Towers, “The Black Gate is Closed”).
Other beings, such as Hobbits, Men and Dwarves, kill and eat animals. Elves, Tom Bombadil and Goldberry appear to be vegetarians, for they are never mentioned consuming meat. And as far as I can tell, Ents only consume liquids and minerals. Orcs, trolls, dragons, and spiders are known to eat the flesh of men (and I’m pretty sure there is a passing mention to Gollum eating babies). References to cannibalism are seen in depictions of the orcs and Shelob. In her endless hunger, Ungoliant devours herself. Monsters consume, but they are never nourished. This relationship with cannibalism, self-consumption and consumption of the other allows us to generalize a monster’s eating behaviors to larger interactions with other individuals.
In Tolkien’s legendarium, consumption is the ultimate denial of the humanity of the Other. Monsters are ultimately self-obsessed, which results in the destruction of others. To be a non-monster is to express pity for others and to exercise mercy to its fullest capacity.
[Not intended to be an actual reflection, but more of an extended comment.]