One of my favourite Preacher stories is the story of the Saint of Killers. I forget what collection it's in, but I remember the introduction where Garth Ennis talks about how the movie Unfogiven played a big role in inspiring the character. Particularly, the stuff at the end where William Muny damns himself. He just outright damns himself. And so does the Saint of Killers.
And here, so does Frank Castle.
In all three cases, these men are put in situations where they have a choice and they all choose to damn themselves forever. And what makes it so powerful is that Ennis understands this and knows how to convey it.
In the final issue of Punisher: Born (I prefer the originally intended title of "The War Where I Was Born") is so powerful and haunting. Some no doubt disagree with the idea that Ennis puts forth, that Castle became the Punisher in Vietnam and that hs family getting killed was just an excuse to let the beast free again, but it works for me. It adds to the character without deviating too far from the way he was originally written. As far as I know (and I could be wrong), Frank Castle always was a Vietnman vet and there's no way that couldn't have affected him. It makes sense to couple that with the killing of his family.
I don't know, but the page where the soldiers find Castle standing among the bodies, that is one of the most disturbing pages I've seen in comics.
The rest of the comic is great, but most of it is just window-dressing for those moments at the end--it all leads up to there and . . . just damn, man. Damn.