The fact that someone actually thought of this is probably just as scary as anything in the movie.
Well, that and the fact that people agreed to act in it.
Plus, the fact that it apparently did well enough to warrant a sequel.
And it's getting ANOTHER one.
This is just one depraved movie idea that actually became a movie. How many more sick, demented ideas have been thought up that didn't make it to actual theaters?
Doubles as Fridge Horror, but, Dr. Heiter's past as the lead surgeon in separating Siamese twins. That's not a throwaway line, that's his backstory. Given how rare it is for both twins to survive the separation (or just survive period), and how common it is for both to die, it's not a stretch that operating on pair after pair after pair of conjoined babies would desensitize a man to human death and suffering and fuck him up more than a bit. Madness may have been his only option.