The way I see it, he broke the rules, and shouldn't expect anything differently.
Also, if I were him, I would've let the robber just take whatever the hell he wanted. To me, that job isn't worth putting myself in harm's way to try to stop a robbery. IMO, he's lucky he caught the robber off guard and wasn't shot in the process of trying to play hero.
You are right of course, you are not your brothers keeper, do not carry a badge and if other people get killed because nobody tried to stop this guy it's no skin off your nose. Sleep tight knowing we are all here looking out for you because you are worth any 20 of us dummies.
It wasn't the store manager that fired him it was the district manager and the store manager apologized to this guy about it. Sounds like he saved a stand up guy only to get knocked down by a petty small minded penny pinching bean counter. Good things jobs are plentiful in this economy, he should have no problem finding another job. In the meantime he can sell his gun to buy groceries and pay the rent with.