It's a one-shot title in Marvel's The End series, which told out-of-continuity final endings for Marvel characters. (Other books in the line showed the fates of the Fantastic Four, the Hulk, Iron Man, Spider-Girl, Wolverine, the X-Men, etc.)
Ennis got the Punisher, obviously, and he did it as Punisher MAX. That should tell you all you need to know.
It's set "soon". World War Three has triggered a global nuclear holocaust. Frank Castle has been in Sing-Sing prison for years, having got too old to stay ahead of the authorities in the way he used to. Now, though, he's given himself a mission. Some people need punishing. Never mind about the end of the world, or indeed the extinction of mankind. Never mind the fact that Castle's already sentenced himself to death just by stepping outside without a rad-suit. Rorschach would have loved Frank Castle.
Is it horrifying even by Punisher standards? Yup. It invents a new kind of "holy shit". The Punisher's choices are chilling, yet at the same time feel true. He's wrong, but he's also right. I admire it. "We died at the hands of evil men... but we did not let it go unpunished."