I don't know if this has been mentioned, but could this have been relating to game cases? He's vandalized the video game section before, right? (xbox sign) Could this be a blue-armed spergging rather than the purchase of pepperspray? Chris isn't one to back down off causes, and according to the facebook post about the mall events, he may feel he needs to be a martyr for his autistic cause. While walmart keeps most games behind glass, there are accessories, and other things in the area that he could have vandalized, though if anything, he'd likely put posters on the glass or something. Again, just speculation.
I understand he was in the hardware department at the time of the call, but it's possible he walked away. I don't see any reason chris would be there other than if he'd walked away from somewhere else to avoid getting into more trouble.