The novel We Have Always Lived in the Castle by Shirley Jackson describes the life of three family members of the Blackwood family who have been isolated from their Vermont community. The novel is narrated by Mary Katherine (Merricat) Blackwood. The three remaining family members have become isolated due to an incident six years earlier where the rest of the family was poisoned. Only Merricat, her sister Constance, and their Uncle Julian survive.
At the beginning of the book, it is clear that the community believes Constance committed the murders. She was the one who always prepared the family's food and did not eat any of the sugar that had been poisoned. However, at the end of the novel it is revealed that Merricat was the one who poisoned the family. It is not revealed exactly why Merricat killed her family. She makes it clear that she chose to poison the family with sugar so that her sister Constance would be spared, but the reasoning behind the killing itself is never given.
One possibility is that she wanted to be alone with her sister whom she loved more than anything, as seen by Merricat's constant desire to protect her sister. Another possibility is that she is mentally disturbed, as seen in Merricat's practice of sympathetic magic—combined with her violent tendencies. However, I personally believe that there is subtext of abuse that has gone on in the house, and Merricat was reacting to that.
Merricat does not like washing or cleaning herself, and Constance comments that Merricat does not brush her hair frequently. Both Merricat and Constance tend to be reclusive—hinting that they have been hurt before. They were also separated from the rest of the community even before the killings through the fence that was placed around the house, which isolated the children. The fact that Constance was in charge of making food, and that Merricat was sent to bed without supper also points to a strict household with very traditional notions of gender rules and enforcement of them.
The subtext of abuse is bolstered by Merricat's extreme reaction to her cousin Charles's visit. Charles functions as an analogue for Mr. Blackwood, Merricat and Constance's father, and engages in many of the same behaviors. The protectiveness the sisters feel for each other could also be a coping mechanism developed from dealing with abuse.