Have you read “the Ultimate Evil” by Maury Terry? It’s a little too tabloid in many instances, but there’s also some solid information there as well.
I think Maury’s instincts were correct (Berkowitz and the Carr brothers were all almost assuredly in on the killings), but the notion that it was a vast international cult (the Process church) that included other groups like the Manson family is a bit harder to swallow.
Some of the pieces boggle the mind. Maury’s belief that Arlis Perry was killed by this group seemed to have been disproven, but there were signs of Satanism at the crime scene and the guy that killed her (the security guard who discovered the body) had a copy of of Maury’s book next to a suicide note he’d written.
The police corruption in this case runs too deep and too much time has passed for the truth to ever really come out, but I think it’s more likely than not the Carr brothers and Berkowitz were in a Satanic group together, and all of them killed at least once.