The parameters of the original scenario said:
"A person is holding a gun on a clerk".
While I appreciate the incredible caution and due diligence many are trying to express here, how this cannot be interpreted as an immediate threat to health and safety is beyond me.
Most BGs are not trained with proper weapons discipline or safety. His finger was probably on the trigger when he walked into the store.
If you are waiting for a sign, it will be the clerks brains splattered on the cigarette rack behind him.