"I mustn´t turn my back on that young man" sounds to me like a father talking about his lazy son rather than being afraid to be shot and killed by him.
I would think that Nevill would ban Jeremy from the farm altogether if he had such thoughts! He would install alarms, sleep with a ready firearm at his side and lock the rest up.
Sorry, but I believe this is one of the myths, which keeps being repeated, but doesn´t mean or prove anything.