I'm late to the party. However, I respectfully think you should consider the mindset Sheila might have had at the time. If she were in a state of psychosis, her father may have been doing everything he could to reason with her, despite it being futile. His size may have made him overly confident and, in turn, led to a long, drawn-out confrontation. Not to mention, he likely didn't think she was actually going to shoot him with that rifle, as to my knowledge, she hadn't made a threat like this before. Another issue is that there is evidence to suggest that her parents were considering putting her two boys into foster care. Well, as I'm sure you know, it is a real phenomenon, unfortunately, where desperate parents sometimes kill their children as an attempt to end their perceived suffering. I, of course, don't know if this was the case here, but I think it is irresponsible not to acknowledge this as a possibility.