Ideally he would have had an interview with Bamber, but look at it this way. Would you object to a psychologist assessing Ted Bundy or Ed Gein without having actually met them but by analysing information already in the public domain? Obviously Bamber is still alive but the information is still there. The law says that he is a murderer even if you don't agree that he is. There is independent evidence to his behaviour before and after the murderers which could be said to be evidence of psycopathic behaviour.
I agree that perhaps June is the key to everything. One of her children was a paranoid schizophrenic, the other a psychopathic killer. We can't say genetics was involved because they were adopted. Was their upbringing vital in the way they both turned out? And was June responsible? Colin Caffel seemed to think so in his book. He had very few nice things to say about June (though got on well with Nevill apparently).
As far as I can see those that have been convicted of horrific crimes either in the UK or elsewhere in the world have been found to have some mental illness/personality disorder as with those you have identified above. Not so with Bamber. Despite ongoing assessments no professional who has actually met with him has ever identified any personality disorder and/or mental illness. I can't see anything in his background either eg anti-social behaviour, violent conduct, etc. Sure there's lots of rumours and myths but imo nothing of substance.
I believe June's mental illness in 1959 is key but I don't believe she is responsible as such for the outcomes. She seems a decent woman and typical of her age and background but I don't think she was a suitable adoptive mother and should never have been approved by the authorities as a suitable adopter. This may seem harsh but Sheila should have been removed from the Bambers care permanently in 1959 when June was first admitted to St Andrews for psychiatric treatment caused by her decision to adopt Sheila (Dr Ferguson's wit stat). The fact that the authorities were not alerted and the fact that the Bambers were allowed to go on and adopt another child beggars belief.
I blame the authorities for approving the Bambers as suitable adopters and Dr Ferguson for not raising the alarm. Perhaps I should add that Nevill seems perfect adoptive father material but he was part of a couple and for that reason sadly as a couple they should not have been approved as suitable adopters.
As you said Sheila was diagnosed as a paranoid schizophrenic; Jeremy has been found guilty in a court of law of slaying his entire immediate family to death and June had two bouts of serious mental illness requiring in-patient psychiatric care. Something not right and I think that something is the key and has been overlooked. Its also interesting to note that Jeremy's full birth siblings are well adjusted individuals. >@@(*&) >@@(*&) >@@(*&)