Yes but there's also the suicide note to explain where "my babies and me go to our rest" and talk of Police arriving, suggesting she is indeed compos mentis. If she's not aware of her actions yet reloads the rifle as Mike is now suggesting there's yet more to explain away.
Exactly. Sheila in her quiet thoughts both on her way to the farmhouse then on the evening of the tragedy must have planned it in her mind and the discussion at the supper table sealed the deal for her in mulling over how she'd been treated in many ways. After Jeremy had left and with supper dishes cleared away Sheila had apparently gone to bed but could have been busy writing her notes until she'd been sure that the boys were asleep and both parents settled.
During this time her inner thoughts would have turned to rage because to my mind you can't kill anyone if you're cool calm and collected so Sheila carried on her mission in a blind panic thinking that this was the best " way out " for all of them. It only takes a moment of madness for anyone with such thoughts to carry out their intentions . Had Sheila known that what she'd done was real ? I don't know ! She'd obviously given it thought especially at WHF where weapons were to hand.