The worst shootings are at schools, malls, and airports.
At schools, Law Enforcement is often too late to save the victims. A shooter can just shoot their way through a window, door, or a single under-equipped unwary officer. They will likely be the first target in the event of a shooting, leaving students and teachers unprotected.
A mall has many unguarded entrances. Anyone could walk in with a bag and pull out an assault rifle or a bomb. Little security. Lots of victims.
An airport has the TSA, but they only take effect when notified of suspicion or when the scanners detect danger. What if a shooter doesn't want to hijack, just to kill as many travelers as he can. Only the planes that carry around 50 people are protected, but not the airport where hundreds can die in an instant.