...it won't be safe for the criminal. There is no guarantee that the clerk will not get a shot off and kill the robber. Handgun bullets cause instantaneous incapacitation under very limited circumstances. They are very good at killing a person, but not necessarily very good at preventing him from killing you, too.
The clerk's behavior is to be commended and is long overdue. We need to stand up to crime and kill criminals whenever they present themselves.
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