I reckon an active shooter standing in a crowd becomes a clear target very quickly as the people around him either 1) get shot and fall down, or 2) run away. There's a lot more than "zero chance".
Though not quite the same situation, my personal anecdote is of an incident a few years back at my local grocery store where a man went on a knifing spree at the exit. He only managed to stab two people before being
stopped by a shopper carrying a concealed pistol. Although in this case, the assailant was not shot and surrendered instead. I would say there were many people grateful for the armed shopper that day.
How about we not have a war on inanimate objects and instead focus on preventing mentally disturbed people from committing random acts of violence?