Again this is just guesswork, but refering to points raised in earlier posts, and borrowing from the film 8mm (i know dragging a hollywood film into a serious discussion isn't the best idea, but it did raise some points that bear discussion)
What if a person (as in the film) was in a seat of wealth and seemingly inscrutible moralty, trust and power (I'm refering to a world leader/politician) someone who would know which strings to pull to keep themselves off most authorities radars. Was perverted enough to fund this kind of film, its intended audience would only be one person, themselves, i think it would be possible (easy even) to pull this off. They themselves would not need to carry out the act, and marketing is not an issue as they don't intend to distribute the film outside their own collection, the film would however exist. and would have been created for thr purpose of viewing pleasure, rather than the need to kill.
I agree that until a snuff film is found and verified, that they will remain a UL. But i also have to agree that its it such a short hop from films that are available that it would be naive of us to rule out their existence completely.
And adressing the point of why the UL exists in ther first place, it may show a general mistrust in the direction of modern life, that our lives and privacy have become so invaded or cheapened (CCTV, reality TV) that we all secretly fear that its only a matter of time before a loss of life (male, female, animal) is an accepted form of entertaiment shrink wrapped and placed on the shelves of your local blockbuster. Fear is the source of many UL's and i would say it seems the most likely in this case.
.... end of what if note: not facts, just opinions.