In the original introduction of The Punisher, King Pin informs The Punisher that Spiderman was the one that killed off his family and essentially gains a free hit man to take the Spider out. By the end of the comic The Punisher learned the truth about Spiderman and instead turns on the King Pin for using him as a pawn.
Regardless of which rendition of The Punisher is your personal favorite...
...Frank Castle will always remain The Punisher.
The aspects of The Punisher that resonate deepest for me though are the aspects of him that are completely human. He has no super powers. He has no money. He is an alcoholic who so happens to be homicidal and depressed. He grieves nearly constantly for his family and it is a reoccurring theme to see him go out of his way to grieve as solitary as possible. Overall he is the most humanized, most realistic, of any Marvel (or otherwise) character that can go toe for toe with others who do have those perks and live to tell that tale.
The Punisher is by far one of the darkest characters of any genre of any media. I'm not saying this lightly either as any of you who know of Jackie Estacado know of how dark The Darkness can get. (Also, anyone who got that reference without having to google it should comment as such below so I can congratulate you on being right on par with my superior level of geekdom) The Punisher takes this to profound depths as he takes down some of the most powerful men in the world in the name of true justice. Some criminals can't be taken down legally, they will still need justice, they will still need The Punisher.