You've got to hand it to the Devil. That guy gets away with a lot. Sure he/she/it is does a lot of bad things but he/she/it doesn't give a damn because he/she/it is the Devil.
Keyser Söze, a Devil figure himself, said "the greatest trick the devil ever pulled was convincing the world he didn't exist." While the Devil can make you do just about whatever he wants there needs to be a human element or vessel involved to carry forward some diabolical plot which is where humanity comes in....
For me, the Devil just makes sense. I feel like his reasons to kill and destroy the population are probably the best out of any of them. He's pissed off so he's going to fuck with God's children. (bare with me on my knowledge of the Christian faith, I was raised by lapsed Catholics) If you were cast out of Heaven and in charge of a place with a bunch of rapists and murderers, wouldn't you want to schadenfreude that shit up? If you say "no", you're lying. Stop lying.
In any event, Satanic films were booming in the 1960s and 70s with the likes of Rosemary's Baby, The Exorcist, The Omen, Don't Look Now and the rise of counterculture. Cults, particularly after Charles Manson, were feeding into the paranoia of the time and the Devil has a way of attacking those most vulnerable; women and children. The paranoia that the United States in particular but the rest of the world felt as well during the Cold War, Watergate and Vietnam brought about the search and need for explanation of the sinister forces in the world.
One of the many reasons we watch horror movies is because we confront a threat and with a bit of luck, the threat gets neutralized. As the general public became aware of Satanic cults studios found ways to capitalize on the sensational aspect of sacrifice and a fully evil entity.
Rosemary's Baby (1968)
Based on the novel by Ira Levin (who also wrote The Stepford Wives) this Polanski classic follows the pregnancy and paranoia of Rosemary Woodhouse and her smarmy husband Guy. This is one of the most quiet and most unsettling pieces of film I've ever seen. It's an incredibly well crafted and watchable film. And while I'm not the biggest Mia Farrow fan, it's worth watching for Ruth Gordon in, what I would argue to be, one of the best performances in horror.
The Omen (1976)
I have a an incredibly fond memory of this movie. My father was so excited to show it to me (I would have been 8 or 9) that he gleefully described the scene where the photographer's head gets cut off with the glass pane then wandered around our house laughing to himself.
For a movie that is that far-fetched it feels very real and addresses the fear of the unknown as Gregory Peck knows it's not his child and must try to put his suspicions to rest. It is fraught with some extremely violent sequences interwoven with a thoughtful and well-rounded script. I always feel like the Omen never gets it's due as one of the all-time great movies. If you haven't seen it, get thee to thine local DVD store and thee can thank me later.
The Ninth Gate (1999)
Polanski and Depp. This should be right up there with the other films I've mentioned here but it ain't. I re-watched this recently and I'm torn between saying the film has little plot and spends over 90 mins trying to make it seem complicated or this plot is ridic complicated and the Polanski just didn't capture it. Either way, the silliness is only amplified by bizarre performances, an uneven plot and ending that almost made me break my TV. Sometimes potential is the biggest Devil of them all.