Are you saying that his conviction is evidence that he's a psychopath? That's a novel idea.
One of the less appealing traits Jeremy Bamber has displayed throughout was and is his attempt to pin the blame for the psychotic murders committed by him on Sheila Cappell.
All who knew her, including her doctor, knew such actions were way outwith her capabilities.
It was important to him for some reason to blacken her name and not only because she was a convenient scapegoat. I think doing that was one of the ways in which he outsmarted himself. It was not only unnecessary but in part it led to his downfall.
He worked at setting the scene for his narrative but he could have chosen to work on a different one. Farms have always been subject to robbery and guns could have been the target. It would have been so easy to set the scene for an intruder disturbed on the job. Would a resulting more stringent forensic examination given him away ... I don't think so.
I think Bamber settled a lot of scores that night and resentment of Sheila was a main one. His desecration of her London flat and his desecration of her memory probably gave him as much, if not more satisfaction than killing her.