Such an intriguing case!
The fact that William Parsons had just been released from prison for providing abortions could very well be related to the murders. And, his sister also worked at the 'chiropractic clinic'. Was someone angry with them for the type of services they provided?
I recently read the book, "Looking for Jane". It's excellent. It's fictional history about motherhood, adoption, unwed mothers' homes and the abortion movement back in the early 1960s (and subsequent). I highly recommend it.
Other possible motives:
- as others mentioned, revenge by the blackmailers from 15 years prior.
- searching medical records (an angry 'partner' of a woman who had an abortion looking for evidence?)
- robbery (drugs?)
- disgruntled family member who either disagrees with the nature of the family business or who is impatiently waiting for an inheritance.