Clearly, you have never read anything not written by Ennis.
Ennis stated before he took over the book that he didn't know much about the character, and it's shown(
http://www.thepunishercomics.com/art...arth_ennis.htm).
Frank got his revenge long, long ago (one of his earliest appearances, also something Ennis ignored). He doesn't kill for that anymore. He kills so no one else has to suffer like he did. It's what the character is about. Read Essential Punisher 1. His biggest rule has always been to never hurt innocent people, and he kills to protect them. He doesn't have any powers. He doesn't have the money or technology of Batman or Iron Man. He kills because it's the only thing he can do in such a world.