He was wearing bullet-proofing so even if every member of the audience was "carrying", that wouldn't have stopped him either. Given that he was at an exit, nobody was sitting near him either.
And yet something must be done. You're saying NOTHING can be done.
Something MUST be done.
Like closing gunshow loopholes and tracking every gun ever made, beginning with the manufacturers, so that transfer of ownership of illegal types of guns can be tracked to an illegal purchaser andremoved from the purchaser immediately after purchase. If cops can see whose auto insurance has expired right away, it's possible to see when any armament has transferred into unauthorized hands.
Umm, in this case the problem was not guns, but the criminal. What part of that do you not understand? How many guns did McVeigh use?
The point being if your intent is to kill people in this manner gun laws are not going to stop you. Remember, YOU TITLED this thread.
So YOU tell us how in the US you stop a mass murderer?