Where are you trying to go with this? That he just woke up one day and said "Man, im gonna go buy an AK and shoot up a bunch of people at the theater tonight, good thing I can buy firearms so easily?" Cause that really doesnt fly, since the things he used to do this clearly constituted pre-meditation, you dont just aquire body armor, kevlar helmet, and wire your home with explosives all in a day. (all of which are highly regulated and illegal)
I'm saying the guy thought it out which means he was acting on very rational thought process, he was clear cut, knew what he wanted to do, and planned it down to every single detail. Point is if any number of details are different it may have reduced his kill count or kept him from doing it altogether.
Logically he would want to kill thousands of people if he could but didn't, because there was no way for him to effectively get this kill count, he was limited, if he could of gotten more explosives into the premises my guess is he would of used them, he is a joker copy cat after all. A medal, detector, a dividing ridge in the buildings structure, faster exits for crowd dispersal, more expensive weapons, better tracking on these weapons etc all could of saved lives.