Undoubtedly, there was some weird stuff going on in Yonkers, but I think that could mostly be chalked up to it being the 70s and frankly, I'd be surprised if some of the more esoteric spiritual elements of the new age movement HADN'T made their way east. But is Berkowitz being truthful when he blames satanists for the crimes to which he pled guilty? It's highly doubtful.
Frankly, this kind of thing is part of David's behavioral pattern. Ever since his arrest, he's been trying to mitigate his guilt by making outrageous claims. First, it was the demonic dogs barking and howling outside his window and telling him to kill. He even kept a diary while in jail carefully documenting his fragile mental state, which he then handed over to Larry Klausner for his book about the Son of Sam murders and if you read that, he really lays it on thick.
Then, a few years later, he'd back off the mental illness stuff and start telling anyone who'd listen (namely, Maury Terry) that there was a cult behind it all and he was just the fall guy, just a patsy (though he would later confess to being involved in SOME of the murders). Terry bought it, hook line and sinker, and then we had a new book with a new story of the Son of Sam murders and their dubious motivation.
In the interim, Berkowitz was interviewed by Robert Ressler of the FBI who saw through his BS and clocked him for what he was: a psycho whose pathology had more in common with an assassin than a typical serial killer. These types are generally fellow travelers of various movements and groups though seldom members themselves. Of course David knew what was going on in Untermyer. He lived there for quite a while. But was he a part of it? No way. But the cult claims stuck and later, when David converted to evangelical Christianity, we'd have yet another mitigation. He very publicly expressed sorrow and gave all the appearances of penitence (though still maintaining the cult claims to an extent, but only because they appeal to evangelical confirmation bias) but again, private letters unearthed showed that he was still nothing more than a wannabe telling people what they wanted to hear in order to get something he desperately wanted, something that all convicted killers want: control over the narrative.
