I have no particular insight into why an 8-year-old boy who apparently was not a victim of abuse would shoot his father and a boarder to death, "methodically stopping and reloading as he killed them." I doubt anyone does, since crimes of this sort are extremely rare:
Kathleen M. Heide, a criminology professor at the University of South Florida, said the odds of such killings "are so infinitesimal, it's really hard to even comprehend."
From 1976 to 2005, there were 62 cases in the United States in which a 7- or 8-year-old was arrested on murder charges, said Dr. Heide, who analyzed FBI data. Only two of those cases involved a child killing a parent.
Which is one reason this pseudo-explanation jumped out at me (italics added):
The boy in Arizona was no stranger to weapons—his father, an avid hunter, reportedly trained his son to shoot prairie dogs—and psychologists said that might have played a role.
Obviously, if this kid did not know how to operate a rifle, he would have had a hard time killing his father with one. But training children to handle guns is a pretty common practice in this country. (I do not come from a hunting family, but I was using a rifle just like the one involved in this case at my Jewish Community Center day camp when I was 10.) If a familarity with firearms creates a substantial risk of juvenile patricide, you'd expect this sort of thing to happen a little more often than seven times a century, wouldn't you? Maybe the step from shooting prairie dogs to murdering your father is a little bigger than the psychologists consulted by The New York Times allow.