My guess was either Muslim or atheist. I was leaning toward the second.
Apparently the kid was homeschooled, and his brother went to a “religious” school, whatever that means. But it could be that he rebelled against Christian teaching that his father tried to give him.
All of this is still conjectural.
Sorry to hear about the homeschooling background. I have met homeschooled rebels but never anyone who rebelled that dramatically. Still when people are given a lot of truth and then reject it they often become much harder and more bitter than those who had less truth to begin with.
They feel a federally funded diet of indoctrination will make them all happy drones...instead of instigators of needless slaughter.