This is a very valid point, and on a packed airliner there would have been persons like that. A gun among the passengers might have found its way to the right person, particularly in the case of the forth airplane. Likewise, a gun, or likely guns) in the school in Breslan or Columbine mmay have resulted in far fewer deaths, by the earlier death of the perpatrators.
We need to move in the direction of a gun knowledgable society once more. A few years back, talk show host Melanie Morgan, (SF) was given the opportunity to learn about firearms, and moved from a scared mom to a knowledgable supporter of 2nd ammendment rights.
Being a gun nut, I talk to people about guns nearly every day.
I meet a lot of people who say they don't like guns.
I never let that slide. I find out "Why?".
Invariably, I discover they have no good reason except that they are completely unfamiliar with guns.
They're ugly, they make a loud noise, and they have the ability to kill.
I always invite them to one of our shoots.
I guarantee them that by the time they leave, they will have a much better appreciation of guns and the enjoyment of shooting.