I think in cases like this, family annihilations, we really need to look at how and why this happens instead of chalking it up to EVIL or DEMONS or whatever. That's just too easy. These situations, like all family annihilations, don't just happen in a vacuum or overnight...they build up over years and years and because it takes place in the home, we have no way of knowing exactly how it evolved. Most crimes, especially domestic, happen in private, and in this case, the annihilator is dead so we can't ask her.
More often than not it's about money. Here's a woman who had four kids, two baby daddies and probably not enough funds, overwhelmed, with few resources. It happens because parents and families don't make great decisions and get hopelessly mired in debt with no way out.
Don't underestimate what desperation can do. As for the kids, in her mind she probably was "sparing them", but we'll never know.