A veteran who had a pistol when someone else started killing people in his university campus explains why he judged it was a bad idea to try to find and kill the shooter.

Several sheriff's deputies waited outside the Marjory Stoneman Douglas High School while the killing was going on, and did not go in.

I don't know enough to judge whether they acted wrongly, but this is more demonstration that having some armed and trained people in the vicinity is no guaranteed protection.