Michael Reeves's 1968 film Witchfinder General is an acknowledged cult classic of British Horror. I don't think it rates as high as other British horror classics of the same period like Peeping Tom or my personal favourite The Wicker Man in purely artistic terms, but that said, I found it extremely disturbing and deeply affecting, despite many obvious objections. At more than a few occasions I even thought of stopping but couldn't do so because it was so powerful. It is gruesome, graphic and unusually sadistic but one can't doubt its sincerity or moral seriousness. The film ends with the hapless heroine screaming in anguish and despair and one feels like adding one's scream to hers too.
If the focus would have been only on physical torture and pain, it wouldn't have worked like that. It is a proof of the artistry of the film that one doesn't realize when physical torture and pain have segued into spiritual anguish, as one realizes the indomitable power of evil. Reeves was only 24 when he made this film and he committed suicide (in dubious and unexplained circumstances) shortly after. This fact, which admittedly is external to the film, nevertheless gives it an added feeling of sincerity which makes it even more disturbing. Did he really believe there was so much evil in the world?, you begin to think. Another factor which adds to the overall effect of the film is the way Reeves used the actual locations. The story is set in the Villages of the East Anglia region of England (the same setting of W G Sebald's The Rings of Saturn, places I really want to visit sometime). The way the spare and peaceful pastoral landscapes are used as backgrounds for the staging of a purely human evil, makes it particularly affecting and unsettling - almost as if human beings befouling the purity of nature just by their presence. It is also interesting to compare it to The Wicker Man in this respect, which is a work of pastoral horror too but which works in an entirely different way.
The Wikipedia article on the film is pretty comprehensive so I won't go into the same details here. As I was watching it I was also thinking of Carl Th. Dreyer's masterpiece Day of Wrath dealing with the same subject - the persecution of witches. Unlike Dreyer's film (which is no less gruesome and horrifying in its depiction of torture), Witchfinder General is not about the nature of faith or religious doubt (or power of human emotions, of course). The titular villain, played by a genuinely terrifying Vincent Price, doesn't believe in Satan, or for that matter even in God. He is just a cynical opportunist, inebriated with power and abusing it to gain what he really wants - money, political favours and sex. It is not surprising that the narratives about persecution of witches have been so easily and powerfully appropriated as political metaphors. Arthur Miller's The Crucible is the most famous and the most obvious example. The word "Witchhunt" itself has taken political overtones signifying abuse of power, systemic injustice built on collective delusion and irrationality and moral panic exploited for political ends. Jim Hoberman in his short review also calls it "an extraordinarily bleak story of political evil." As the description might have made it clear, this film is clearly not for everyone's tastes but those who are looking for something unusual and very strong may like. It certainly should be much more well-known than it already is.