From bits and pieces I've read all over the place it sounds like he was armored up and took pain-killing vicodin... Since the plan evidently was to "give up immediately" to the cops... who was the body armor and pain-preventative meds for? I think he thought there WAS going to be an armed movie-goer.
That's my
While we don't know for certain, early reports mentioned Holmes said he took vicodin to "calm down" while waiting to implement his plan. I think he didn't want to appear jittery with anticipation, so took the drug. I have to wonder about his state of consciousness during the shooting with a combination of adrenaline and vicodin fighting for dominance in his body.