I hope resurrecting an old thread is o.k., given that this post will be relevant. Last year, TCM ran Vincent Price films during October, which also was horror month. Anyway,
Diary of a Madman was one of them, & as I watched, I noted the inspiration was GDM's
The Horla, which I found on Gutenberg, & reading it, saw that (as I now recall) the film seemed a bit more descriptive than the book, at least about the ending, anyway. As it was a very short story, the film did a good job of retelling it.
But in the film, the bonus was doubled, as the voice of the Horla was Joseph Ruskin, the same guy who was in Star Trek's
The Gamesters of Triskelion, while VP's butler was portrayed by Ian Wolfe, who was in two other episodes, one of which was All Our Yesterdays.
Anyway, as I recall, the Horla took possession of VP's character when the condemned man whom he was visiting in his cell, attacked him, and in defending himself, he killed the man. As the days pass, the Horla becomes increasingly dominant over VP, who was, as I recall, a magistrate or some other high-ranking judicial officer. The Horla wanted a young girl, with whom VP had some dealings, and of course, that meant eliminating her boyfriend. I think I will leave it at that.