It's a pretty straightforward situation. The guy was confronted by multiple opoonents, as such lethal force was perfectly acceptable. A lot of people who are anti-gun, or anti-violence really don't seem to "get" how force escalates. Another similar situation is if the person carrying the guy/weapon and responding with lethal force has reasonable cause to believe his opponent has combat training. A guy who knows karate, or has trained with the military can be considered armed and dangerous.
Of course this is a very general breakdown on things, laws and the continuum of force vary from state to state and town to town. Seems like the law was clearly in support of what he did in the area. Some parts of the country are a LOT less friendly towards criminals than others.
Back when I took Criminal Justice, we covered Connecticut's laws (at the time) where the basic situation is one where someone is required to flee before they use any degree of force at all. The only time someone can defend themselves is literally if they are run down or cornered. This exists largely as a way of dealing with rowdy drunks and the like, since it means in a fight, who threw the first punch is irrelevent. If there was a fight and neither guy tried to run away, it doesn't matter, both are liable. There are exceptions to this in practice of course, but at least how the law was then this guy would quite probably have run into trouble because he didn't try and run away after being punched before pulling his weapon. Of course if attempted escape was impossible, or he was boxed in, what he did would have been fine also since he was outnumbered. Simply put he's not a cop carrying a utility belt full of options, taking on two younger and stronger people hand to hand would have resulted in him being killed or seriously injured, he likely didn't have any options between "fight barehanded" and "blow their heads off". With "fight barehanded" not being a viable option... well, that's what the gun is for.
I understand people who are anti-gun wanting to spin this into some kind of travesty, but the bottom line was that a pair of scumbags tried to rob the wrong person, their ages being irrelevent to the situation here.