My mistake re Wilson's recollection about NB's remarks having been put before a jury. Nevertheless, weighing up such remarks as those attributed to Sheila are simply be cast aside by Steve as 'believing she was a white witch'.
However nowadays, as any professional who works with vulnerable or high risk people would tell you, if such a person makes claims about death or killing... and then goes on to carry them out, there would be hell to pay, possibly via a serious case review. Which would demand an explanation as to what actions had been taken regarding the original comments having been made.
So Steve's dismissive, white witch throw away comment, in my mind, belies a malevolence on his part in respect of the defence / Jeremy Bamber.