There are numerous "sensible gun regulations" on the books now.  Don't know what you want to do with "nuts" and "mentally unstable people" however.  I guess you could pass laws making it illegal to be "mentally unstable" or a "nut."  But, you likely wouldn't kinow that until after an "event."  

This particular "nut" was smart enough to avoid every single video camera located in the hotel, parking facility, etc. as he carried in however many firearms and the ammo needed to feed them as well as his personal effects.  

There are all sorts of mass killings, like tornadoes, hurricanes, floods, etc. and we are lucky that we have have well-trained workers to handle these events. 

Regardless, tragedy often brings a clearer idea of a person's "job" in this world.