I’d say “8mm” draws on idioms and archetypes of the pre-net porn world. Before it was possible to access such pinnacles of human achievement as goatse and 2G1C, it would have been plausible to think that such things WOULD be going on in some shadowy “underground” world nobody “normal” knew about. In the age of the 'net, pretty much the one thing left for this sort of distro would be that which portrays actual illegal acts (such as CP – and if the same pattern holds as for similar crimes against children, it’s likelier for the culprits to be trusted members of the family/neighborhood circle than some gang of white slavers…)
Pretty much by the reasoning that anything that is illegal or universally socially censured does not disappear but instead becomes clandestine, there has always been underground porn (as there is underground lit), but, it ALSO it has always had a mystique that it’s a bigger and worse conspiracy than any of the mundanes can imagine. This is kinda understandable in an ontological sense, as a precursor of net-age’s “Rule 34” (“There is porn of it. Whatever ‘it’ is”) . If you can imagine that there is someone out there so sick he gets his jollies from X Y or Z atrocity, then OMG, sexual desire is so mighty and overriding someone must be seeking to make it real and “they” – the ever inevitable “they” – must be running a network to get new victims and get away with it!
One has to realize that some of the perversions we consider a sign of how bad things are today are nothing new. If you read De Sade’s Julliette or The 120 Days of Sodom, you’ll find that (1) much that he was writing about 200+ years ago would be on and beyond the edge WRT bizarre illegal sexual conduct today AND (2) he already was imagining networks of the rich and powerful getting together for rape/scat/torture/sodomy/blasphemy/pedophilia/murder orgies in their chateaux and abbeys.
Thing is… there are serial torture-killers in the world. But is it plausible that “Hostel” describes a common ocurrence?
I boldfaced your own answer to the question. See also “carny geek”.
The more bizarre part of it is that in any case Deep Throat set the wheels in motion for the rise of “legit” hardcore… which still involved a lot of unsavoury characters and behaviors (already referenced, John Holmes’ linkage to the Wonderland incident) but appearently no gun-to-head scenarios.