According to the local police, who IMO are very, very squared away, Holmes bought a ticket to the midnight showing of TDKR, and during the first Catwoman scene (people who watched the movie know what scene - I haven't seen the movie), he pretended to go outside the theatre and make a phone call. He propped the door open, took out weapons that he had hidden, threw a smoke grenade of sorts, and started the carnage. This means he walked up BEHIND everyone while he was shooting.
Police responded within 90 seconds, as stated in the news, and he exited the theatre through a side exit door. He stood outside the theatre waiting for police to show up. They are unsure, because he won't talk to anyone, but they believe that he may have wanted to "off" himself, but didn't. He told them that he was the Joker (or something like that) when they arrested him (without incident). The police put bags over his hands to keep the firearm powder residue intact (smart and sharp), to trace to the weapons, and this guy acted like a clown and made puppet scenes with the bags.
"Some people want to watch the world burn." - Michael Caine as Alfred
Nothing is truer than the way this guy is behaving. Had he not told police about his booby trapped apartment, the death toll could have been much, much higher.
My personal opinion: There's more evil to come related to Holmes.