...yes?
Let's accept, for the sake of argument, that arming victims does in fact reduce or eliminate the ability of a mass shooter to inflict casualties. The only way to ensure that someone is carrying a concealed weapon in whichever screening is hypothetically attacked is to ensure that someone is carrying a concealed weapon in most if not all screenings. That's a lot of handguns.
The vast majority of these weapons will be used responsibly, but some small fraction of them will be used in crimes of passion or suicides that would not have happened but for the easy availability of a deadly weapon. Some other tiny fraction of them will injure or kill someone accidentally.
Mass shootings and terrorist attacks, while incredibly tragic, are incredibly rare. They also rarely have casualty counts above the low double digits. Meanwhile firearm accidents kill hundreds of people every year, and suicide and homicide claim tens of thousands. Arming ourselves to a level that can prevent black swan events is optimizing for the wrong case and will almost certainly lead to more loss of life than it prevents.