Tactical cops - those in ESU or SWAT may receive far more training than the beat cop, but they too can end up firing and injuring bystanders.

In a mass casualty event, they would need to clear the movie theater and anyone with a firearm would be considered a threat until cleared. That means they’d waste time trying to deal with all the good guys with a gun before they actually stop the bad guy with a gun. In that kind of situation, every moment counts, so if you’ve got a handful of good guys with guns firing in every direction (because they’re all firing at each other in a confused manner since they may not recognize which is the true bad guy with a gun), you’ll likely end up with even more casualties.

Again, this isn’t the movies. The reality is that more firearms added to an already volatile situation will result in higher casualties.