I’ve read that the theater was a “gun-free zone,” i.e., a zone where citizens are legally disarmed against sociopathic freaks. It’s also been reported that there were members of the military (with gun training) at the theater during this midnight showing (some of whom would have brought guns, if the theater allowed it.) Even with the tear gas, if there were multiple military at that showing, it’s likely that one of them would have got off the shot, preventing some or most of the fatalities and non-fatal injuries. Thus, the movie theater’s “gun-free zone” policy is a likely cause of some of these fatalities and injuries, most particularly the ones that occurred later in time, when the element of surprise - and the tear gas - dissipated, at least to some extent.
All of this is the predicate for a negligence/wrongful death suit against the corporate entity that owns the theater. That corporation, or its agents, corralled the victims into a forseeably dangerous situation, made dangerous by that corporation’s idiotic “gun-free” policy.
Depending on one's job in the military such "training" might have consisted of learning to identify the trigger and which end the bullet comes out. Not all positions require expertise with weapons.