a good guy without a gun..
France high-speed train.
Louisiana State Trooper (who had a gun) but was surprised by and killed by a bad guy with a gun. The bad guy was stopped by passing motorists, neither of whom had guns.
or a bad guy with a gun..
NYC - A man forces his way into a federal services building and shoots a guard (who I believe was armed). He then walks into the building and stops himself by .. committing suicide.
All of these cases point out that bad people get guns, they get them far too easily, and yet, rarely are they ever stopped by people with guns in hand. That's just fantasy.