Having doors that can be secured could backfire. A shooter could already be in a room and make it difficult for a would-be hero to enter and take out the murderer. I am sure a shooter is fully capable of using the lock too.
And, the intended victims are fully capable of throwing the bolt open on the way out. Even in a worst case scenario, by the time the shooter exits the room, every other room within earshot has been locked down. The damage is largely contained to that one room.