If you believe guns are reasonable and useful for defense (and I don't), then that is what they are for: defense. To prevent greater harm from coming to pass through threat or use of force.
They are not for killing people because you think they did X or Y. That would make you a vigilante.
-Jester
edit: typo
They are not for killing people because you think they did X or Y. That would make you a vigilante.
-Jester
edit: typo