Sheila's behaviour latterly was " all over the place "-------unpredictable,one minute okay,then such as with Freddie,manic. That's how Sheila was !! I maintain that she had snapped !
I'm glad you mention Freddie. Consider how this passage from CAL shines clear light.
"With detectives at Witham leading the investigation, Freddie Emami was driven to the police station there for an interview. He acknowledged that
Sheila had had ‘a very quick and violent temper which she would lose over the simplest of things’ but had never seen her use physical violence towards anyone. He ended his statement:
‘Had it just been her [adoptive] mother who had been killed I could accept it, as
she disliked her intensely"
June was the one she shot first, then everything followed like dominos toppling. Nevill. Then the kids. Then Sheila. This I believe, no grand plan at all.