There are a LOT of would-be mass murders which were cut short because someone was armed and stopped the attacker either before killing started or after just 2-3

There are a LOT of would-be mass murders which were cut short because someone was armed and stopped the attacker either before killing started or after just 2-3 deaths. They don't enter the mass murder statistics precisely because they didn't happen despite intention thereto. The info you want is easily found on the web.