While the SDA does not give authorization to intervene on the behalf of a non family member, I'm pretty sure it gives you the right to intervene in a semi-proactive fashion if you feel your life is in danger. If a guy in the same room as I am in whips out his piece with the intent of using it to harm someone/getting what he wants he would be indirectly threatening me. If he shoots the clerk, I'll be willing to bet he would be willing to shoot me. In that case I would act and eliminate the threat.