Americans have this weird romantic notion of vigilante justice. I guess it ties in with the mystique of the Wild West or something, but even our depictions of that are largely fictional. Those settlements were considerably more civilized and law abiding than portrayed in movies and TV. Things like the OK Corral were the exception, not the rule.
But anyway, while I think everyone understands the anger of the father in this situation, you can't go around meting out your own personal brand of justice. I think our legal system is a largely broken joke of an institution, but I still prefer it to a bunch of people running around pretending they're The Punisher.