I think he probably did go too far, from a defensive standpoint.
I don't know this for certain, having no experience in the matter, but it would seem to me that a person defending his home (or any position, for that matter) has tactical advantage. It would also seem to me that if you give up the cover of your home to pursue out into the street, you've given up cover, concealment, and all the other tactical goodies that come from defending a position as opposed to attacking. If he's beat them out of his home, I would think the prudent thing to do is call 911, and hunker down until the constabulary makes its appearance. Pressing the attack doesn't seem to be the best thing to do.
From a legal standpoint, cases are easy to win when there's good guys and bad guys. When you can point to someone and say definitively they are the bad guy, the contrast makes things easier. This homeowner, rightly or wrongly, chose to step out of his home to continue the fight. As noted, there might be some legal justification, only time and all the facts will tell. However, he did blur the line to an uneducated citizen/juror as to the good guy/bad guy equation. Pressing the attack in this case now has made him harder to defend, legally. From a valiant homeowner, he can now be portrayed as a bloodthirsty vigilante. That's the angle I'd take if I were prosecuting him.