I'm inclined to believe that both elements were employed. Charlie/Hoyt probably had a mental disorder. Leatherface probably did as well besides the physical deformed features. (Ie the wearing of the mask). In the original 1974 film Drayton Sawyer played by the late Jim Seidow had schizophrenia. I'd rather the family be evil and leave it at that. As much as I enjoy movies like Chainsaw, Halloween, etc I don't really like when the filmmakers employ the mentally unstable card.