Fighting back in the defense of another is not vigilanteism. The judge was right to call him a hero (his employer called him a hero as well, but denied encouraging such behavior) as what he did was heroic. However, heroic actions always carry with them a great risk and this young man paid the price. Everyone knows this. This article is missing a lot of information and even a little misinformation. The Man did not "slap" the woman he full out punched her in the eye according to one source.
I do not see anything wrong with breaking up a fight, but I would not have "tackled" or shoved this guy out the door like the employee did. Also, calling the police first would have been the best option. Perhaps it happened too fast. I don't know, they don't give enough information about the circumstances leading up to this event.
We are not vigilantes, we are not LEOs. We carry a weapon for only one reason - to defend our life or the life of a loved one. Period.
I disagree with your blanket statement. There are times when defending others around you, especially when it also means defending yourself, are the right course of action. My firearm is for defending whomever I choose to defend within the legal realm of the law. To say that I have to defend everyone around me is wrong just as saying that I can only defend myself and my loved ones.