The tragedy in Colorado was unavoidable in terms of the having ANY way of preventing this previously seemingly sane young man with no criminal record from obtaining the guns and bomb making material.
However, the fact that he could buy a ticket, enter the theater and then sneak OUT an UNGUARDED exit and PROP IT OPEN for reentry with his lethal arsenal to wreak havoc was certainly avoidable.
What theater, especially one showing a first run expensive film like this would not have precautions in place to prevent people sneaking in without tickets? An ordinary common precaution to prevent someone from buying a ticket and then sneaking in non paying friends would have prevented this tragedy.