Even before reading the article, I was predicting that the woman was psychiatrically ill. The article notes that she has a history of mental illness, though the specifics of it aren't yet clear. My guess is that she was suffering from some kind of delusional disorder in which she was convinced that she had to take her son's life for some reason (e.g., to protect him from eternal damnation - kind of in the same way that Andrea Yates thought when she drowned all five of her children). Auditory command hallucinations may have been involved as well. Of course, in trying to figure out what really happened, I've got a ton of questions:
-What's the extent of her history of psychiatric illness, and what exactly does that history include? If my hunch is correct, then she likely has a history of severe psychiatric illness punctuated by psychotic symptomatology (e.g., delusions, hallucinations) that has required emergency treatment (e.g., involuntary psychiatric hospitalizations).
-If she has a psychiatric history, then was she compliant with medication at the time? If she does have a psychiatric history, and psychosis did have something to do with this, then I'd bet she wasn't compliant with her medications.
-How often do they go to shooting range - was this something they did often, or was this the first time? That she had to rent a gun makes me think this wasn't something they did often (or she had a gun of her own that was taken away because of her psychiatric illness). Where I'm going with all of this is aforethought: did she use the firing range outing as a convenient context in which to murder him and then herself? After all, she had to know that she would have virtually unrestricted access to both a firearm and him in a vulnerable position (i.e., looking down the range away from her) at a firing range.
This case, to me, while certainly tragic, is absolutely fascinating from a forensics perspective. It's been a while since I consulted the research, but, if my memory serves me correctly, instances in which a mother murders her own (i.e., biological) children is exceptionally rare, and those cases typically involve psychiatric disturbance.