It is reasonable to assume that murder was the motivator, the side benefits of murder being a bit of larceny, robbery, perhaps some sexual aspect, re Miss Deese, voyeur-related, ( the incel aspect perhaps a profound part of that ) firesetting, ( this is very much a part of the trademark of serial killings, and it wont be a surprise if it comes to light they both were bedwetters up till an advanced age, 10 - 12 , another signifier ) .
The yarn about work was nifty, who can object to two young men going off to find work? . One can hardly say one is off to kill folks.
Is their own death part of the plan? I don't think they have saved the last two bullets for each other. I believe as long as the maximum amount of carnage , damage, wreck , ruin , destruction , death and despair can be activated, that's the plan.