Notice that planned mass shootings tend to occur where people are unlikely or not allowed to have guns.
I don't mean to turn this into a gun control discussion, but there will no doubt be calls for banning guns again. That horse left the barn in the USA over 200 years ago though and isn't going back in.
I'm not saying everyone needs to be armed but if 2 or 3 people, just 10% of the victims, had been carrying and could shoot back, then many lives could have been saved.
I'm normally carrying concealed and no one would ever guess, which is how I like it. And it's something I never thought of until a few years ago when some whack-job in Pittsburgh woke up one day and decided it was his day to shoot Jews and immigrants. I'm neither, but one place where he killed some people was WAYYYY too close for comfort. If it ever happens again, I'm shooting back.