David Bain: A Motive Emerges

SHOOT: It's possible that Robin Bain was going to be off work for a protracted period, and that period would have started on the Monday. David Bain would have been painfully aware of 2 things:
1) The incest of his father with his sister was likely to increase
2) Arguments with his father over the chainsaw were likley to increase
3) Stress at home in general would have gotten worse

David may have seen his father as a milestone around his neck, making it difficult for him to meet other people or appear successful. being the principal of a school would have made the embarassment worse, not only for David but his sisters too. His younger sister was also a head girl who often fought with her mother.
David may have decided that killing them and blaming his father would allow him to escape the torture of humiliation (associated with his dysfunctional family). I think the fact that the murders happened on a Monday is significant.

clipped from www.nzherald.co.nz As she was going through files she found a document signed by Robin asking to be relieved from his duties as principal because of stress. She recalled the period of leave sought was approximately within months of the killings. She thought it may be April, 1994. Asked by defence lawyer Michael Reed QC what had happened to the document, she said that was a good question. It may have been filed again, but it was not thrown out. "I don't know what we did with it." Her understanding was that Robin was asking her to relieve at the school from the week of June 20, 1994. This is the day of the murders.