This kind of reaction to this situation is similar to how we see poor threat analysis in any variety of situations.

There is a certain logic in responding to the massive focus that we see on these deaths, by a reaction that says let’s just isolate this problem and find a solution. Let’s arm everyone, and a shooter like this won’t be able to take down so many people. You respond to the immediate threat.

What is missing is a measured response, that takes the time to think through the follow-on ramifications of arming everyone - more accidental shootings, more shootings done by the mentally ill, more suicides. In the end, any advantage gained by taking out a mass-shooter sooner would be negated by more gun-related deaths.

The poor analytical thinking crops up in many threat assessment situations, such as the illogic behind invading Iraq in response to 9/11.