My analysis of the situation is that he probably had personal issues with the people running both stores, or perhaps even with the management of the mall, with Gamestop simply being the place he chose to start the fire.
All comments about anti-gaming advocacy, and insanity aside this seems like the kind of move that would be performed by someone with a legitimate, but irresolvable greivence. Most disgruntled employees who realize there were legitimate reasons for them being fired/not hired by others/blackballed rarely do anything. On the other hand righteous rage is more likely to lead to events like this. In a general sense I tend to think less incidents of this sort would take place if there were more viable recourses availible to people wronged by employers/potential employers.
I could be wrong here, but in cases like this there usually isn't much doubt as to the motivation, especially if the person surrenders. Even the insane have motivations that make sense to them, and if the police talked the guy down, some information must have been exchanged.
I look at situations like Heemeyer and his Killdozer, which is noteworthy because the incident was so over the top that the authorities couldn't really sit on the motivation.
In general the more reasonable the act (even if illegal) the quieter they tend to be, because the authorities are concerned about encouraging people to do similar things. It's pretty much unheard of to find an article that takes the side of someone who goes on a shooting rampage or whatever for this reason.
Understand I don't know anything, that's just what I think, based on other cases I'm familiar with over the years, and how this played out. If the guy was a nut or had insane motives, the authorities would be giving that to the media to run with in many cases. You'd be surprised at the lengths people will go to when greviously wronged, or who have their lives ruined, and there is no viable recourse open to them.
Yeah, I'm extremely cynial when it comes to these kinds of things.