If Shelia was indeed completely out of it as Adam suggests, it all points to a snap decision been made that Tuesday afternoon possibly after an argument. And done out of hot blooded anger.
I seriously doubt if he was being methodical and calculating he would have done it if he knew and he knew other people knew the condition shelia was in.
The overkill of a six 4 foot 16 stone farmer and a woman who couldnt get " off a sofa" as an alternative suspect isnt a perfect crime.