I think Neville had totally given up the fight.He'd had enough,was tired,weary to say nothing of being unhappy at the way things had panned out all round. He probably felt that he couldn't do right for doing wrong. He certainly wasn't aware of Sheilas' chronic condition,,as he'd only taken onboard what Dr Ferguson had said about it being " mild " schizophrenia. You can't have mild schizophrenia.! But nobody,,not even the psychiatrists realised or knew at the time. They don't know enough now in 2013,,so there was no chance back in the 1980's.
If psychiatrists back then had warned Neville of the dangers of such an illness and its consequences,,I don't think Neville would have been as complacent,,or Jeremy,if he'd been told.
It was a total lack of awareness of the illness and its repercussions that was wholly to blame for the tragedy,,,and the very fact that we haven't moved on much further with understanding the implications,that I find disturbing,,,as murders/suicides are still happening today involving those who are diagnosed as being paranoid schizophrenics.