Sticking more to the Knapps — without researching their lives (and accurate evidence is not going to be in a NYT article), I would venture to speculate that:
1) The kids started smoking marijuana and drinking at young ages.
2) Their public school was a dope marketplace.
3) They ingested the trash culture Hollywood uses to make billions out of “heroes” like “Snoop Dawg” and “Eminem”. Lots of video gaming and worthless layingabout.
4) When they first started drugs /drink, they stopped maturing, leaving them with the maturity and prudence of 10 to 12 year-olds.
5) As irresponsible, immature slackers with no foresight, they had no ambition, no desire to succeed at anything, no work habits, no developed skills, etc.
6) Leftist schools taught them that lack of success was someone else’s fault. Just read the NYT.
7) Being given a hot car at age sixteen gave the oldest boy complete access to any sort of vice.
8) Their addictions, petty crimes, and poor work habits made them unemployable, even if evil capitalists had job offerings going unfilled.
Good observations.