The decaying mental health of the protagonist, and the way he treats the poor cats, is very unsettling.
"Gentlemen, I delight to have allayed your suspicions. These walls — are you going, gentlemen? — these walls are solidly put together."
And, of course, what they find inside those walls...
The 1934 film:
The skinning scene, which despite the Gory Discretion Shot is among the most disturbing moments in a pre-Code horror film, and one that manages to frighten people decades after the film's release.