Even in Ohio, where (in the last few year), concealed carry has reached 1 in 20 adults, most don't carry them everywhere, many of the spots it happens at don't permit it, and there are actually whole webpages devoted to crimes foiled by armed citizens, the left just sticks its fingers in its ears and pretends they don't exist or handwave them away. We don't have many spree killings, and for your argument to be anything but absurd we'd need to only count the cases where someone present was an armed citizen.