I've got two scenarios about how B and K left PA (and a few clues as to what the "triggers" were for their sudden homicidal trip, would love feedback):
1) They were sent off with cash and a fond good-bye from their families (with an open door to return any time they wished)
2) They were told they needed to get jobs, make some money, and get themselves out of the house, somehow (with the truck and camper being the stepping stone). If they decide to come back, there will be rules.
Since, where I live, #1 is certainly common enough, parents often do send along cash, as though the kids are going to summer camp in the Big World. But often, #2 is employed.
In both scenarios, the parents are trying to get the kids to launch. The kids are notably inexperienced in the real world (for B and K, not even a recent, real school experience and apparently very little work experience).
Both can be experienced by the teens as being "pushed out the door," but the 2nd is the more tough love approach (although still involves permission to take the truck).
Being sent out of a house which had electricity and a fine gaming computer to live in a camper (sure, it's your best buddy, but you're both used to having your own beds, rooms and real showers)... can feel jarring. If there's underlying mental disorder, that stress can really bring out the poor cognition.
We don't know for sure if drugs were involved, but sudden loss of a lifestyle that possibly included sleeping whenever one liked, gaming online, gaming outdoors, not much work-life, electricity, running water, someone else paying bills etc...might have been really been hard on one or both of them.
If both were suicidal and depressed before they ever left home, then once on the road everything seemed overwhelming and they became a ticking time bomb. If I were a betting person, I'd bet they had alluded to suicide, to each other, on more than one occasion before leaving PA.
The picture of BS with the gun in his mouth speaks to that. You could even say that rather than kill their parents/family members as some youths do, they struck out at "others:" those who were clearly able to launch. The last message to their families shows that family was still a priority, somehow (but not strangers, absolutely no emotional or ethical concerns about "others" at all).
Ultimately, it was aa really brutal thing to do to their parents, grandparents and siblings.