Most think that the attraction to gore (not Al, that's learned behaviour) is innate.  If we can generalise, perhaps it is a fascination to see the effects of things.  I don't think children pull the wings off flies because they are innately cruel, but because they want to see what happens.  And when that effect is on the body or makes deep incursions into the psyche, it is all the more fascinating. 
It's also fascinating to watch fish swish about, to watch the wings open up on a beetle, and to get the second theme in a symphony. 
Indeed, I think most of what's wrong with the world, and perhaps some of what is good, comes from the interest in having an effect on things.  Lust for power and real cruelty that seems to be exterted without cause can be explained by this principle.  It's guided my understanding of many people's behaviour, and has helped me to gain the necessary detachment from it.
If you like literary references, consider Dostoyevsky's Notes from the Underground, where the narrator's notion of freedom comes very close to this idea.

Hi Arthur and Gregory,
Interesting  what you say about how people are interested in effect, which fits well with your example of the animals in your Do animals have rhythm post , but what would you say about freaks in the circus? The bearded lady, people with long fingernails, the giant, and so on. There is nothing having to do with effect there, it seems to me. Maybe one could say it has to do with possibilities - seeing possibilities of bodies, possibilities of what the body could become? Maybe, actually, you could say the effect which is interesting there is the effect on the crowd, the effect of the disfigured body on oneself, which is what people want to feel? Perhaps.
Or maybe, the effect they are interested in is simply being disgusted, like in horror films, feeling scared, horrified?

I must say about myself that though I find the figure of the monster very interesting philosophically, as I am too, as you describe, very interested in effects and hence the effect of the monster on people, I'll admit that myself, I am not at all interested in seeing monsters. Disfiguration, outside of Francis Bacon which I would more describe as a disfuguration of the world than of the person, has very little appeal to me. Why is that then?
    

To summerize: what you describe answers part of the question, but not really the fascination with looking at extremely fat people, circus freaks, and accident injured people. I think feeling the possiblities of the body, and the (horrific) effect of the image on the person looking is also part of the answer (as is the wish to feel one's own body which is so hard for many people nowadays), but not all of it. I think there is much deeper psychology going on here. (or, if not deeper, at least another facet of people's psychology.)

This conversation reminds me of a book I read by Jules Michelet  “The witch”, it’s perhaps not completely related but that was my first association with the subject. In the first part of his book, Michelet paints  a portrait of an era and of a fictitious woman. Through her he explains the phenomenon of how the being of the witch came to life and how for the common populace, they were both the terror and the first provider of help. I won’t go into details, he basically relates the hardships of the people living in the middle ages when Christianity became the ruling religion and there was a rejection of antiquity’s traditions and of nature. He writes on how the creation of little devilish spirits and of witches came as a necessity, a liberation and a gift of life. It gave a possibility to get out of one’s misery and become rich and happy or to perish (as can be seen in all the fairy tales of those times), it gave the people hope and an opportunity to get out of their sad reality. I thought it might be close to what people are looking for today, many  are suffering from boredom, another sort of death, and need to feel  alive and reconnect with nature.