sounds like built-in comedy to me
IMO there is no such thing as an accurate depiction of psychopaths. The PCL-R is based on experiences with prison inmates who are probably only the low functioning end of the spectrum. Who knows what the other end is doing. I was happy with the characters of the movie simply because they don't fit the stereotype.
I more had the feeling that that scene was meant to artistically cross the uncanny valley.
How to make that funny? Either lower the quality into B-Movie wonderment or find a way to have us not only not sympathize with the victim, but believe they deserve it. Present enough buildup to make the audience wish for it and present the plot and the torturer in the right way and people can find themselves laughing. Beyond that, the whole "Comedy = Tragedy + Time" notion could at least make it funny in hindsight.
The Joker has multiple times been a commentary on the idea of trying to make one's suffering funny, but depending on the writer it's pretty hit or miss.