According to my English teacher (1970s), the real definition of the word "nice " is "correct", so obviously, I'm using it incorrectly! (Funny how some things teachers say stick in one's mind many years later).
You are right that we can't really know someone without spending time with them (and I've no interest in getting to know JB, as he won't be the person he was in 1985). And I also agree that the evidence is what is important, not what sort of a person he was.
But, it seems, he was a liar and a thief , stealing from his own family's business. He was also unfaithful to his girlfriend, and liked to use money to show off. So, in my book, not a particularly pleasant person, although this does not necessarily mean he committed the murders.
I recall a former boss who started off his career as an English teacher and his pet hate seemed to be the word 'nice'.
But most of the main protagonists do not come up smelling of roses:
JM was happy to assist JB with the caravan park break-in. She was also happy to sell JB's homegrown cannabis on campus. And she was happy to carry out other crimes independent of JB eg cheque book fraud and using cocaine.
AE comes over as greedy and grasping. Her husband, Peter Eaton, was reported to the police by farm secretary, Barbara Wilkinson, for financial impropriety at WHF.
CC by his own admission in his book let SC down in many ways. He eventually left SC and the twins for another woman when the twins were 5 months old.
You point out JB was unfaithful to his girlfriend but given he was unfaithful with her best friend what does this say about the best friend?