I think the part we're missing here is that anybody can go to the airport, park, stroll into baggage claim and open fire. He didn't have to come on the plane. I've checked guns and ammo many times when I fly. I generally don't bother with the ammo anymore as it's not enough to get through a match and just send it all ground. It can be in a factory box or reloader's box, either is fine as long as it's not over the weight limit. In all the years and times I've flown, I have never been asked for my permit and have only had a couple of nasty people from either the airline or TSA.
Often the gun case just comes down with the rest of the luggage, which I don't like. Some airports are more careful and you have to pick it up at the baggage office by showing the claim check or they call your name on the loudspeaker and ask you to meet the baggage person at a specific location. I like that better.
Even moving security to the entrance to the airport won't help, he did everything per the rules and would have gotten on the plane, gotten off at his destination, gone to baggage claim and we would have the same result.
The issue is how did a mentally unstable person - and a vet to boot - end up not getting the help and attention he needed before he went totally off the deep end and killed a bunch of people?