What is so horrifying about the Cauldron Born, from Frankenstein to Voldemort? It's not that they're alive, nor that they're human. The problem is that they're not fully human, that they're masquerading as human but something has gone monstrously wrong. Something is lacking in these would-be humans, and that lack makes their powers frightening.1 I think the appearance of cauldron born beings in literature -- engineered by human hands -- explores our discomfort with the limits of what we can make through science or magic, and how that falls short of the life found naturally.2 It mirrors back our unease that we may not, after all, know quite enough about what we are doing to be able to fully handle the results. It is, in its way, similar to the story of the Fall all over again: we have a temptation to godlike power and, with more knowledge but less wisdom, it becomes a dehumanizing and nature-corrupting mix with results that are difficult to predict and more difficult to stop. The same basic theme informs much of the environmental movement's fear: that our knowledge and our greed for gain have outpaced our wisdom, opening up the potential for catastrophe. In more moderate terms, it's a critique of the history of human attempts to control the world, including a critique of certain applied sciences, where history has proved a good few times now that our knowledge of how to do something has outpaced our knowledge of the consequences and side-effects that happen when we do it.
Close cousins to the Cauldron Born are the creation-gone-haywire themes. In recent times these have focused (unsurprisingly) on computers. HAL (one step ahead of IBM) from 2001: A Space Odyssey is always a favorite. So is the unnervingly dispassionate military computer who responds to a teenage hacker's request, "Let's play 'Global Thermonuclear War'" in WarGames in a way that threatens to destroy the planet.
These man-made horrors also reflect the old theological observation that evil does not have a nature of its own, but takes another nature and parodies or corrupts it. Evil's nature is defined by what it lacks.
But it's not a right Halloween post if I spend all the bandwidth explaining why the genre strikes a chord. Let's get back to the Cauldron Born. Nobody else I've read does the Cauldron Born quite as sickeningly as Rowling's Voldemort. For Halloween, and for the identity of evil, sickening can be a mark of having shown just how revolting evil is ... right? ANYWAY ...
It was as though Wormtail had flipped over a stone and revealed something ugly, slimy, and blind -- but worse, a hundred times worse. .... no child alive ever had a face like that -- flat and snakelike, with gleaming red eyes. ... (Skipping a piece of black magic done on behalf of the villain ...)
But then, through the mist in front of him, he saw, with an icy surge of terror, the dark outline of a man, tall and skeletally thin, rising slowly from inside the cauldron. ... Lord Voldemort had risen again. (J.K. Rowling, Harry Potter and the Goblet of Fire, various excerpts from chapter 32)
1 - It's debatable how much the "fundamentally inhuman" bit applies to Mary Shelley's Frankenstein and the movie adaptations. Is the problem in the created object, or just in our perception of it? Is the problem that we go beyond what we can reasonably handle, or is the problem that people can't handle "progress"?
2 - I know that not all of the "man-made humans" in literature turn out too badly. There's Pinocchio, whose life is not frightening to others, but personally sad until he becomes more fully human. This is much like the Tin Man in the Oz stories. But those get demoted to a footnote for Halloween! Tonight we explore the dark side of the story.
General footnote: Yes, I get impatient with those who do not see horror and science fiction as "worthy" or "serious" literary genres. They are the myths of our day where we grapple with the questions of life, death, and our place in the world -- or the universe. Granted that not all horror and sci-fi are high quality, still the same could be said fairly for any other genre.