They like to think they are writing their own script, they don't want to copy cat anyone.
Phase 1 of this adventure: They didn't exactly plan to be spree killers, they joked about it (but I believe one of them had a plan all along). However, they planned as if they were spree killers. In adolescents, this is not really considered abnormal thinking or "play" but it can deepen.
Phase 2: They planned an actual crime (with out much real attention to detail). They were going to rob someone with camping supplies, probably targeted lonely turn-outs, found the couple in the van. One of them opened fire - for thrills, to do it, to be really badass, to impress his friend, to be as crazy as possible. (Watch something like Murder Mountain for a look at people who drift off in this outlaw direction - although in that film, the badasses are ones who managed to survive their teens by quite a bit).
Phase 3. From that point on, they knew they were "dead men" and had now entered a new reality, similar to leveling up in a game. Whether they have the survival skills to stay alive off the grid...as opposed to just dying out in the north...we may not know for a while.
One of them might break off from the other. They may get a second wind (but I'm guessing that the "high" of their initial spree has left them exhausted, the bugs are getting to them, not having showers/toilets - and not having computer access - is all getting to them. If they remain relatively exhausted and lost, that may become their new "survival gaming."