I think that is possible, not likely, but possible. (One of my theories on the Missy Bevers case is that it is a younger adult.) But to add to your theory, what if we have a young adult, 18-19 yo, who has graduated HS and living at home. His parents go on a cruise or other such getaway and junior has a job so he stays at home while their gone. He uses the truck and his father's .357 (or .38 spl or other such caliber) revolver. Might the parents come back and not be any wiser? My own father had 2-3 handguns, but some he had not fired in 5-10 years. As long as the guns were cleaned and reloaded he might not pick them up or look at them for another 2-3 years.I've considered the killer may have been a younger adult, possibly still living at home, who borrowed his parents truck. When LE came calling, the parents hired an attorney and the investigation stalled. Related to something to do with Star Wars group.
JMO, just one theory.
I don't think it is likely, but with what few details we have I can't rule it out.