Another ridiculous "argument' is "people would have been killed in the crossfire!" Let's say you're there with a 9mm and someone runs in front of you as you pull the trigger. Tragically, you kill one person then take out the shooter and save 10 lives and 30 wounded. The ONLY headline the next day would be, "Vigilante kills innocent movie goer with concealed weapon." Not a word about lives saved or injuries prevented because to do that would be to admit guns can SAVE lives.
You know why the murder was so low in the "wild, wild west?" Because you think twice before getting froggy when the other guy has a Colt .45 strapped to his leg, too. Criminals love unarmed victims. Tyrants love unarmed citizens.