He worked long hours on a farm.
Maybe seeing shelia and the twins lolling about on the farm while he was sat on the tractor stemmed up a lot of anger and resentment. That very Tuesday. Especially if he has been planning to kill them. The straw that breaks the camels back.
" I'm here grafting my arse off, she is relaxing and getting money for nothing " " I can't take this anymore "
I have always said if gulity that I don't especially believe monetary gain was the main reason. I believe if gulity he was just oozing with resentment and hostility. A lot of anger and felt he was being dealt a bad hand.
In those days men didnt talk and show feelings. It was frowned upon.
I think if gulity Jeremy had fantasised long before meeting Julie of ending his family.
Perhaps something from childhood, perhaps abuse from greshams, perhaps something even more dark.
As Jeremy maintains his innocence, it will forever remain locked in one mans head and we can only draw our own conclusions.