If you feel that your life may be in danger and you'd be overpowered, but you have a weapon, by all means, use the damn thing.
The problem is that in these situations, there's rarely enough time for someone to guage whether or not defense through weaponry is necessary. It's also important to note that most people's aim isn't very good. I'd much rather disable an attacker than kill them, but I'm lousy with a gun.
Most of the people who arm themselves are unfortunately paranoid motherfuckers who would shoot someone for lifting a glass of water. Those are the last people who should be carrying a weapon.
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