Supposidly I heard, that in Texas, for it to have been Home Defense, he had to wait till they were inside before he could shoot them.
If for example he shot them on the balcony or outside, he would have been in the wrong, and possibly gotten in trouble.
I could be wrong, or have misunderstood the rule, but if that's the case....
I get a feeling this old guy knew the rules pretty well, especially given his past, and waited till the right time to balst these a-holes trying to mess with him.