For too long, the attitude has been that private citizens should be passive in the face of crime, running or hiding.  "Don't be a hero" was the mentality.  But we need heroes.  It was the heroic passengers of United Flight 93 on 9/11/01 who prevented the plane from slamming into the U.S. Capitol.  It was the heroes on the French train who prevented a massacre.

In some cases, running or hiding may be the right response.  If the professionals are on the scene, it is best to leave it to them.  Other times, though, the combined efforts of multiple people, even if unarmed, can end the killing, and more companies are now including active response in their active shooter training.  Michael Rosenwald has this story in the WaPo.