I'll try to argue for the other side. It'll get quite hypothetical but honestly this is already pretty hypothetical. A mass murderer's goal is to kill as many people as possible, especially easy with large groups of defenseless people. Non-mass murderer people defending themselves likely didn't want to kill people in the first place. In the event a large majority of people were armed, that would deter mass murderers in the first place, as they wouldn't get anywhere without getting shot very quickly. The problem of mass murderers probably wouldn't even exist in the first place if so many of them were unsuccessfully stopped by armed people. In the event a mass murderer entered say a crowded theatre, and a lot of people are armed, the mass murderer would shoot indiscriminately with the aim to kill. Someone defending themselves would shoot down the mass murderer, and even if more people pull their weapons they would likely recognize that the situation was over if the person who just successfully defended themselves stopped shooting at anyone else. No one else is a crazy mass murderer and won't continue the shooting. It's probably not the most reliable source, but still an interesting study in human psychology. If you're familiar with a game called Trouble in Terrorist Town, it sort of has this premise. The goal of the traitors is to secretly and successfully kill all innocents and get away with it before other players figure out who the traitors are. No one knows who's innocent and traitors. 90% of the time in a properly played game even when two players get in a gun fight and one lives, other players even in the immediate vicinity who witnessed it do not automatically shoot the person who lived, they question their own judgement first. This preservation of the life of others and judgement questioning exists even in a virtual and very hostile world where their own death doesn't even have real consequences where you'd think people would sacrifice themselves and shoot on a whim anyways, and it likely would play out similarly in a real life situation because naturally most people are good and do not want to kill everyone, but the traitors and the mass shooters do. The premise and conclusion of this side I think is that it's better to have a chance at defending yourself than no chance at all.