Watch the series Soft White Underbelly on YouTube. Could be any one of a million different ways people's lives go off the rails in some fashion or another. Sad thing is most of those people are homeless and J&J Crumbley seemed to be cognizant enough to pay bills and take care of business -- up to but not including realizing their son was turning into a homicidal lunatic.

As has been discussed there seems to be a fair amount of substance abuse in this household by both parents. I would wager that their upbringings are similar to the people shown on SWU in that common themes (lack of one or more parents, suffering abuse as a child) will likely be the case with either or both the Crumbley parents.

Interesting to wonder if Ethan has found some way to extract himself from that situation would James or Jennifer ultimately continued down the road of self-degradation until they too were destitute on the streets. Because it seems like the other child, Eli, was capable of seeing how monstrous Jennifer was and removing himself out of that mess. Seems inevitable that destruction was final destination one way or another.