The 1974 film adaptation of the crimes of Ed Gein, not necessarily made as a movie inspired from them like Psycho or The Texas Chainsaw Massacre, but actually attempting to be an accurate depiction of the real events. And it was the most accurate movie up until the 2010 Ed Gein film, so why is it likely the only movie I've brought up (blog post) in this writeup you haven't heard of, even if you've seen the others? Let's find out, shall we?
Source: Deranged Movie Review - Decker Shado