I think what you did was perfect....its probably what I would have done as well....You didn't jump to conclusions and become too quick to draw and be a hero, instead you went to investigate, sounded like you did it while being tactically sound. I will correct you on a common misconception, you mentioned protecting yourself, your family and your PROPERTY.....the first two, spot on, however, you cannot use deadly force to protect your property.
As for yer question about protecting others, you are also allowed to use deadly force to protect someone else from serious physical injury or death.....However I will ad this, in the scenario you gave, if we modify it a little and make it a robbery of the Plaid....I would say it would probably better to be a good witness then to engage the bad guy.....Most robberies (Especially conveniant stores) are ussualy non-violenent. Most of the time they demand cash, get it and leave...obviously this isnt always the case and sometimes you just gotta make that split second, gut decision, in the heat of the moment. Sometimes pulling yer gun will esculate the situation even further, and unless you have a good shot, may not be the safest for anybody else inside. Another thing to consider, is say you blast the robber, saving the stores $57 from being taken, and potentially saving the clerk.....Yer justified so you dont get into trouble, except now robber's baby momma hires a wrongful death attorney and decides to sue you.....They paint you as a crazy gun nut, trying to be a hero, who took some innocent, misguided young persons life, who was only stealing to feed his family
Even if you win, you'll still be out huge money for yer lawyer to defend you....just something to think about.
I have ran this and similiar scenarios in my head....if I see a robbery and am not in the line of fire, I.E. concealed or un-noticed by the bad guy....I will try to be a good witness for responding LE, get as good a description as possible of suspect, this assuming I do not believe the clerk is in any serious danger. Now once the bad guy leaves, I may pursue to get a direction of travel, vehicle description, etc...Now once outside if no one else is around, I may ID myself as LE and attempt to stop suspect, depending on cover, if suspect has put weapon away, safety, etc...Now if I engage suspect in a gun fight outside, at least I have elminated any potential bystanders....I will ad this tho, I wouldn't recommend the pursueing and confronting an armed suspect..