I think it is because only a psychopath or someone with some type of personaluty disorder or mental illness could have perpetrated that crime. Jeremy therefore HAS to be of that ilk because these people have already decided he's guilty. Jeremy Bamber has a sense of humour. Cold blooded inadequate people lack humour.
I think Maggie that it is a debatable area. Have you actually seen the reports that say he is not a psychopath? Who were they done by? under what conditions? What kind of test was performed?
The answers to these would make things a little clearer. Why would someone need to get checked 27 times? This has always puzzled me.
Incidently psychopaths do have a sense of humour. What they lack is empathy, sympathy or any sense of guilt. They do not feel any pangs of conscience when they commit atrocious crimes. However, they do feel anger, frustration and have a very normal sense of humour. Ted Bundy apparently had a great sense of humour.