I'll try to make it clear.
1. You tell me that x said y. I check x's statement and I can confirm that x did say y. What he said may be true or untrue, but he definately said it.
2. You tell me that y said x. As there's no statement I can't check whether y said it or not. You could have made it up, disremembered y's exact words or even disremembered who said it.
1 has a checkable source and 2 doesn't. Therefore 1 can be given more weight than 2 can.
Although I agree with your post- wholeheartedly. People can deduce from someones behavior and form an educated opinion. JB gave his diagnosis with his sister. Mental health can change hour to hour in patients.
The fact the JB claimed his sister went berserk/had a psychotic moment as she was prone to- he claims -so is therefore responsible for the killings, where as the medication she was taking was sedentary in effect.
To say the psychiatrist have given evidence about her diagnosis in court, they did NOT claim she was guilty of such crimes. And to be fair as I have said before, the diagnosis is like an MOT only relevant from the last consultation. AND further more they were not present at the time of the murders. not much weight can be given really.