This is simply your opinion. If you have any concrete proof of it it would be interesting to hear. It would also be interesting to know how "nous" equates with psychosis. You appear to be suggesting that she'd kept secret for years a love of guns. WHY? Are you also suggesting she'd planned it for years?
I'm not suggesting that Sheila had a " love of guns ", nobody in their right senses would have. Neither am I suggesting that she'd been planning the killings either .
All I'd suggested was that Sheila, as a child growing up had appeared to have had more about her than Jeremy as regards common sense and learning and after having spent years on the farm would have watched her father load a gun or two and may have even been by his side when it came to shooting vermin-----we don't know.
DB had admitted showing her about guns in their earlier years, she was always surrounded by them on the farm.