If Sheila did it, what we don't know is if she intended to kill everyone, including herself. Maybe one killing led to another in her mind.
There are three motives that overlap
Motive A) Believes the twins are possessed by the devil, Hatred against June, and also wants to end her life. No known hatred for Neville however in the house he is an obstacle that needs to be eliminated in order to kill June, the twins and herself.
Motive B) The threat of having her children taken away from her combined with her parents suggesting and supporting the idea. This creates the motive for altruistic filicide and could also create a dislike of Neville.
Motive C) She could also have killed the twins in an act of what is know as spouse revenge filicide. Wanting to make Colin suffer for one reason or another by killing the children.
Mothers, particularly, are the group with the highest risk
of becoming victims of homicide by patients with schizophrenia
(Devaux et al., 1974; Estroff et al., 1998; McKnight
et al., 1966). In this study, the rate of matricide was also
higher than the rate of patricide, and female sex of the victim
was a significant factor that raised the risk of parricide
20-fold. Raising a child is primarily the mother’s responsibility
and they are the caregivers who spend the most time
with the patients. Thus, they play major roles in forcing
medications, forcing hospitalisations, and providing discipline.
In this process, long-term conflicts between mothers
and patients can occur, and mothers can be direct targets of
longstanding delusional ideas of persecution or paranoid
ideation or both.
source- Clinical features of parricide in
patients with schizophrenia (2008)All of the women in the NGRI group described psychotic motivations for
their murders. Common themes included the delusional conviction that the
child was defective or monstrous in some way (such as possessed by Satan, or
half human and half dog), hallucinatory commands to kill the child, and the
idea that the child could be saved from disaster (fates such as being raped,
becoming a prostitute, or undergoing torture) only through death. In contrast,
the women found CR described a variety of nonpsychotic motives. Two of the
eight (25 percent) indicated that the child was simply unwanted; three (37 percent)
asserted that the child died by accident, in the course of a beating; one
(13 percent) reported that the child died after an accidental fall; and two (25
percent) blamed another for the death.
source- Insanity and Filicide: Women Who Murder Their Children
Carol E. Holden, Andrea Stephenson Burland, Craig A. Lemme
In this case, the reasons for attempted suicide and extended
filicide seem to be two: (1) altruistic in the light of concern
about the welfare of the children and (2) spouse revenge in
the background of marital problems and infidelity, and intent
to induce pain, sorrow, and guilt in the husband by taking
away children. The demographic characteristics and the
background history are similar to common features reported
in the literature such as lack of social support, primary care
giver status, and relationship problems between the couple.
The reasons for not giving any hint may be spousal infidelity,
wanting to take revenge, depression, and lack of knowledge
about availability of help.[6?8
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Source - Filicide as a part of extended suicide: An experience of psychotherapy with
the survivor. M. Manjula, C. R. Chandrasheka"Her complaints during her prior hospitalizations included command hallucinations
(which told her to kill herself), depression, agitation, and paranoid
delusional ideation. She was being followed by Community Mental Health and
was taking antipsychotic medication at the time of the murder.
The defendant was evaluated on the issue of legal insanity approximately
five months after the murder. At this time, she described the voices as suggesting
that she kill her daughter and indicating that her daughter would be
better off and happier dead."
Insanity and Filicide: Women Who Murder Their Children. American Journal of psychiatry. Published in 1996.
