Even if you had a gun, you'd have to pull off some pretty badass stunts to put down that sucker. The article said he had several hand guns as well as a shotgun, and an assault rifle.
ON TOP of that he had a riot helmet, and body armor, protective clothing and eyewear. So basically you would most likely have to get the drop on him and make it count.
Witnesses also spoke of thick smoke, maybe he had some kind of grenade.
Most people going to a movie theater would never think of getting in a fight much less shot at, this guy chose his target carefully so that he could obviously inflict more pain and suffering while meeting little to no resistance.
It says he was also a Neuroscience Phd candidate, which makes me believe he probably had some sociopathic seed in him already and something in Colorado just brought it to fruition.