This is no excuse,,but there were two psychotic women under the same roof. I've worked on a ward full of them and this is where the word " bedlam " comes into its own. It was sheer Hell,with them screaming at each other,all with the same twisted personalities.
Sheila,to my mind,had her personality manipulated by her mother from a very early age,as I doubt if Sheila would have gone down the same route as her mother if it hadn't been drilled into her that she was a " child of the Devil ",,something that the girl/woman had to try and shut out of her mind over the years,but was constantly reminded of.
June herself was a sick woman with her religious psychosis,which in effect is a form of schizophrenia,after having suffered long bouts of depression herself to which she was treated with ECT for. Nobody fully recovers from the illness,they just remain in remission until such time as an emotional issue crops up then the illness kicks off.
Because of Junes' religious issues,she may not have taken any medication which to her,would have masked her feelings towards " God ",,as this is something which does happen in those of a religious bent. Certain types of medication make you forget the thoughts you have,,but June wanted to be in full control of them.
This was NO opportunistic massacre,,it was between two women whose beliefs spilled over into a tragedy.