Then why would he attempt to frame her? It's his nemesis who has claimed she couldn't perform this or that simple task - and as you say, the notion of her killing everyone doesn't make sense. So why did it make sense for Jeremy and why did he rely on it making sense to others?
Because he got wind of the remark she made to June and Barbara Wilson round the breakfast table: "
All people are bad and should be killed.". It was manna from heaven for Jeremy who had feigned interest in the Farm for his own evil purposes, but this interest only brought forth the offer from Nevill of farming Little Rentners Farm, done for the best of intentions but would just have entrapped his son in a lifestyle he had not chosen and felt no empathy for. Sheila had had a similar offer from June of managing an antiques shop in the locality, but again these were the parents' ideas of what would be suitable for their children and not the aspirations of individuals who had been spoiled, who didn't understand the value of money and through Nevill's long hours and June's illness never bonded fully with their parents.