I think you just answered your question. Normal people just don't do this sort of thing. As a mother of three, whose house STILL breaks out in complete chaos when my grown kids come home, I never came close to considering the possibility of murdering my children either. And Heaven help anyone who DOES try to hurt them.
Can you imagine what must have been going through the mind of the seven year old boy? The living NIGHTMARE of his own mother, his protector and nurturer, STALKING him through the house...who the heck could he turn to to save him? Certainly not his mother.
Psychotic or not, every time I think of those kids, I would cheerfully STRANGLE the b*tch (if I could reach her).