I'd heard that this was a proto-Devilman, but it's more complicated than that. The original 1971 manga was the most successful of Go Nagai's early career. It was cancelled prematurely and deemed too violent for TV, but Toei asked Nagai to create a human-like devil superhero. In other words, Devilman started out as the family-friendly version of Dante.
There's a 1994-96 shoujo manga version (WHAT???) and a rewritten 2002 version that finishes the story. This anime was made at the same time as the latter, but doesn't use it. Roughly the anime's second half is original.
Anyway, the series was inspired by Gustave Dore's illustrations for Dante's Divine Comedy. Nagai took a fondness to Lucifer, portrayed as a "huge creature stuck in the ice in the center of the Earth". He was getting attacked for outrageous content in his manga, to which he responded by turning the devils into heroes, to provoke the "keepers of the standard" and show that good and evil could be slippery.
It starts conventionally. Ryou is a normal young man, except for mysteriously surviving a motorcycle accident in ep.1. He has a nice sister (Saori) who likes coming into his bedroom in a tiny nightdress and asking him out on dates. He also has a father who looks like a caveman and is the leader of a committee to do God's work.
Unfortunately, serial killings of women are taking place. The devil's apostles are abroad! There's plenty of gore, outrageous demon monsters and surreal massacres. For quite a while, the show adheres to conventional morality. When Ryou gets eaten and possessed by a satanic lord of hell, he's unhappy. Seems reasonable. You might be too. At this point, my main problem with the show was its Satanists, who are a bit pathetic and never as interesting as the demons themselves. Ryou also seemed to be holding the show back, by trying to deny his demon nature whereas Devilman embraced it.
After a while, though, you start getting puzzled. Why have the holy men created their own pet devil to send out to kill random women?
Eventually, you'll see the full depths of Nagai's lunacy. God is a petty, mass-murdering control freak and an alien invader. He's the Daleks, basically, but more powerful. The devils are hardly paragons either, c.f. black masses and human sacrifice, but at least they have a reason for wanting to commit genocide and wipe out the human race. I'm not sure I agree with it, but I can't deny that it's wild. (If we consider Jeremy Bentham's "greatest good of the greatest number" principle, I think weight of numbers would be in mankind's favour. Well, that's devils for you. They're not fans of democracy.)
How does this compare with Devilman? To some extent, it's an unfair comparison. Dante's not just a Devilman dry run, given its unique mythology and crazed approach to Christian theology. Their apocalypses are different. As for the core cast, the Saori incest undertones are something with no echo in Devilman's Akira-Miki relationship. I don't think the Ryou-Medusa relationship really works, but that's deliberate and part of her tragedy.
Sometimes it's laugh-out-loud silly, e.g. God's giant robot at the end of ep.12. Sometimes it's grotesque mass slaughter. And sometimes it's going full-bore Nagai, in which all bets are off and you're expecting more extreme storytelling decisions in the next 25 minutes than most writers would manage in a full series.
It's definitely worth attention. Odd and not entirely successful, but it was groundbreaking at the time. Ryou's not that interesting, although sometimes he's startling. Saori's adorable. I'm very glad to have seen it, anyway.
"If I killed, I'd be stooping down to God's level!"