I have several explanations which range from "lolwut" to "ehh, maybe":
(1) Having grown up in San Diego, he was exposed to the mind control devices and
biological experiments emanating from Camp Peddleton!
Have you seen the strange round buildings there with their own private docks?! THEY"RE CONTROLLING US!! AHHHH!!
(2) James Holmes was tired of seeing the same content on main stream media, so he decided to "change the channels."
(3) James Holmes was operating with SPECTRE in an attempt to distract the nation while SPECTRE moves in for the kill. ETA of SPECTRE Coup d'etat: IMMINENT!!
(4) James Holmes was encouraged by the other mass killings (the Columbine-esque ones). These shootings provide a means for some to identity with, and seeing that it was possible, he realized that he could conform through such means.
Interesting facts:
-James Holmes asked about a gun which was ideal for home security about 2-3 months before the shooting. (It was at some porn store, which I didn't think sold guns).
-James Holmes dropped out of a PhD program. (Neuroscience? Neurology?)
-James Holmes moves to Aurora, CO a few weeks before he implements his plan. (Why Aurora? Was it mere happenstance, was it the relative lack of potential police response?)