Frankencastle. For the uninitiated, Frank Castle, The Punisher, was killed and then brought back as a Frankenstein monster. Sure, at first glance, it looks like they've ruined the character. But the whole thing is an enormous middle finger to the "comic books have to be serious" crowd. That isn't the brilliance. Think about it. He died and was brought back with spare parts and mad science. If it weren't for the neck bolts and head stitches all the fans of XTREEM would think it was HARDCORE. HE'S BASICALLY A BAD-ASS ZOMBIE CYBORG NOW, just he's based on Mary Shelly instead of the Terminator or Cable. The writers managed to give the hardcore comics fans what they wanted WITHOUT GIVING THEM WHAT THEY WANTED. That takes skill.
Kicks in when you read the one-shot The Tyger, which reveals that when Frank was a boy, a girl he liked was raped by the son of the local mafia don, who made a habit of doing this to local girls because no-one would touch him due to their fear of his father.
The fact the Punisher rarely encounters superheroes above the "street level", like Spider-Man and Dare Devil, and usually only kills D-lister or particularly silly C-lister supervillains makes a lot of sense, when you think about it. The Punisher only really works because he's pitted against human-level opponents; Suspension of Disbelief doesn't allow someone who is explicitly "a normal, if insanely determined, man with lots of guns" to believably stand up to the likes of Cain Marko, Omega Red, Magneto, etcetera. And while Frank's sanity has come increasingly under question, nobody could accuse him of being stupid. Frank knows he's out of his league against the high-tier villains, and so deliberately avoids their radar — if Frank kills one of Norman Osborn's goons, the man will ignore him. If he goes after Norman himself, then he'll get the full attention of the Green Goblin. Similarly, most of the heroes ignore him because, even if they despise him as a person and/or loathe his methods, he's usually far less of a concern than the latest world conqueror or planet destroyer or whatever the present big bad of the week is.
Taken to its logical conclusion in Dark Reign, where the Punisher tries to take on "Dark Wolverine" (Dakken). And, though he puts up a hell of a fight, is ultimately cut to pieces by his instantly healing opponent. Furthermore, Dakken was sent after Frank because Frank broke his own rule: he tried to assassinate Norman Osborn, who currently basically runs America on top of his usual antics.