Oh wait I learned about stuff like this in one of my psych classes. *Grabs Sociology book*
Ah yes, here it is. I'll start off with an example:
There's been studies to see whether there's a connection between smoking and lung capacity. This research showed us that there was a positive correlation between smoking and lung capacity (meaning the more you smoke, the higher your lung capacity is). This is of course ridiculous, since we all know people's lungs worsen when they smoke. But what the study didn't take into account was age groups; The older you get, the bigger your lungs get and the more you smoke. Once the researchers examined all the age groups separately they found a negative correlation between smoking and lung capacity. In this case there was a third variable in play, namely age, which -at first sight- tricked us into believing smoking would increase your lung capacity. This is called a spurious correlation.
Now, back to the matter at hand: I believe what we have here might be a spurious correlation. There seems to be a connection between shooting people and playing D&D, but if you ask me it's far more likely that people with a psychological profile like her are more likely to play games such as D&D. Which is in fact interesting to know because if you can piece those different traits together (history of violence, reclusive, etc.) it could help law enforcement to do their job better in the future (pinpoint more exact location where a suspect may be hiding, what kind of actions they'd undergo, etc.).
So I don't see it as "D&D made her do it", even if that's what's implied in the article. I think it's an interesting piece of the puzzle that is her mind, which can prove useful.