Between chores this morning, I googled to ask if the theater in Aurora was a gun free zone. In 0.13 seconds, I got 750,000 results. Turns out, Cinemark Century Theaters is in fact a gun free zone. The company, standing firmly on property rights, has a policy that no one can carry a gun in their theaters. So, while the red-headed sonofabitch knew that Colorado is a concealed carry state, he also probably knew that the theater didn't allow law-abiding citizens the right to protect themselves from murderous evil.
So, while Roger Ebert wonders why no one shot back, the answer is sparklingly clear and Roger would have known this if the took 0.13 of a second to Google it. No one shot back because the law forbid it. The theater is a gun-free zone and law abiding citizens check their freedom at the door.
There's your problem, right there.