Well mass murders don't happen in a vacuum but are a product of many years of hatred or abuse bubbling under the surface. The trigger can be something relatively trivial: in Jeremy's case it may have been when he crashed his car and expected a replacement, a boardroom row or the refusal to repay Nevill the £2000 loan he had received for his New Zealand trip. David Bain had also crashed an expensive motorcycle on a test drive and had a row with his father over the use of a chainsaw.
So the same could be said for a trigger for Sheila perhaps her mother And the abortion , calling her a harlot, the childcare , drugs , etc etc .
If you are going to say a trigger could be trivial it could apply to her as well .
And Jeremy probably had no idea that she had said she was worried she was a danger to her own children so how was he going to cover those murders in his plan ?