I watched "Demons of the Mind" after not having seen it since it originally appeared. My memory of the film was very positive, and there are some interesting ideas in the script. However, there are an overabundance of plot elements that are presented in a haphazard and overly hysterical form by director Peter Sykes. One other reviewer here calls this a free-form narrative, but for me it was a confused jumble.
Robert Hardy plays (or overplays, as others here have noted) Count Zorn who is convinced that there is madness and other evil in his family's bloodline.
His wife had committed suicide, so he decided that he needed to lock up his children in case they started manifesting any insanity. Years later he has a controversial doctor (played by Patrick Magee in his usual mannered way) treating both grown kids (Shane Briant, Gillian Hills).
At the same time there are young women being brutally murdered in the woods and local superstitions are being whipped up, while a wandering evangelical (Michael Hordern) mutters religious dogma and joins with the locals.
A good director could have woven all these piece together nicely and provided a solid, disturbing thriller. But Sykes is more interested in whipping up a lot of intensity in each scene, which is why there's more overacting than needed and why the film winds up becoming exhausting to watch after a while. Too bad. It had the makings of a fine film. Perhaps the usual rushed schedule that Hammer Films had didn't allow for sufficient care, though screenwriter Christopher Wicking had history of penning horror films that were more interesting in concept than in execution.