In California you can't shoot a fleeing robber. In Texas, you can.
Keep in mind that in all likelihood this was not the first burglary these two had committed, and had they successfully fled the scene without injury, this would not have been the last. They would have laughed about the close call and about beating up the 'old man' and promised each other to be more careful the next time.
IMO, Greer did society a favor.
No doubt. But there is the old expression that some people are alive just because it's illegal to kill them. And scum or not, it's illegal to just shoot someone when it isn't a case of self defense.