The is absolutely no legitimate solution to prevent any of this.
More gun control? Won’t help.
Less gun control? May be slightly better, but still won’t help.
Make it so you have to go through metal detectors to get in the theater? Ticket prices sky-rocket, or another govt’ agency is created to fuck this up too (see: TSA). Plus, he went through the emergency exits.
Armed guards in each theater? See above.
As everyone can see, this killer was very adamant about doing this. He rigged his entire apartment prolly because he knew it’d be searched afterwards. His parents or family may have been able to do something since they knew he was fucked up.