Cinemark knew, or should have known, that the exits could be propped open. That's the language one typically hears in a negligence case. Cinemark has 522 theaters with 5,888 screens, so I have no doubt they employ a corporate security officer whose job is to make sure things like this don't happen, or, at the very least, to try and mitigate the harm. It doesn't look like that person did their job, as a certified whack-job was able to defeat their security.
As for the movie-goers, what more could they have done to protect themselves? They weren't allowed to carry (per Cinemark's own rules), so Cinemark assumes a higher standard of care in that case. Perhaps the victims should have been wearing some Level 4 body armor, if they truly wanted to feel safe. Yeah, let's put the onus on the victims...
BTW, I was mistaken about the security on hand that night. There was none. Other Cinemark complexes did have guards, but not at Aurora.
I know that exits can be propped open. That should be obvious to any observer. Yet I go to the theater quite aware of that security issue. So does everyone else. Yet they still go.
I think someone else opined about the no guns policy similar to what I think about it. I usually ignore the policy and carry anyway. I've never had an issue but if they noticed and asked me to put it in my car or leave, I'd leave and ask for a refund if I've already paid. If signs carried the weight of law, I would agree with you that Cinemark would carry a higher burden to protect moviegoers.