How do any of us know that there wasn't a fierce argument after JB had left that night ? We DON'T do we ?
With Sheila's vulnerability brought about by the horrendous life she had, it would have culminated in a heightened and impulsive anger and being surrounded by guns made her " job " in ridding that which caused her situation,easier. Without any planning,her reaction would have been instant and carried out on impulse,as suicides are.
Just the split from her husband would have been enough on its own to tip her over the edge,but being ill,helpless in looking after her sons,unfit to work and at loggerheads with her mother,it would take a strong person to overcome what Sheila endured as well as the thought that she was losing control of her sons.
It reads the same as a list of warning signs leading to suicide.FACT.