(or "Ix-Nay On The Estroying-Day The Urface-Say Orld-Way!")
When Stan Lee and Jack Kirby created the Fantastic Four, arguably one of the world's greatest comics magazines (for evidence to this effect, see "Fantastic Four: The World's Greatest Comics Magazine!", Lee, Stan and Kirby, Jack) they made a conscious effort to tie in the modern Marvel Universe to the Golden Age Timely/Atlas universe. It makes sense from a sales stand-point (kids might have heard about the Timely heroes from their parents, and be curious) and from a writing stand-point (the more characters you can throw in, the better the chances are that one of them will spark a story idea.) And since there was a new Human Torch in town, and since one of the old Torch's sparring partners was the Sub-Mariner, it's no surprise that Subby turned up pretty quick.
Of course, when you've already got four protagonists in the book, a new character works much better as an antagonist. So in no time flat, a rationale was worked up to explain why the Sub-Mariner (a good guy in World War II, albeit a spiky, arrogant one) became a bad guy in the modern world (he was ticked off because nuclear tests destroyed Atlantis while he was off being an amnesiac homeless guy.) And before you can say, "Hey, where'd you even get a giant bipedal whale from, anyway?" He was off to conquer the surface world.
But something kind of strange happened along the way to Namor's becoming a big-time super-villain. He became kind of, well...cool. Popular. Readers responded to his tortured nobility, his romantic gestures towards Sue Storm, and his habit of betraying the bad guys when they crossed his code of honor. He was sort of the Angel/Wolverine/Dinobot of his day, and as with so many anti-heroes and noble villains, he wound up getting his own book.
Well, his own back-up feature. There, his adventures focused on a long-term, epic struggle with his own warlord Krang for control of the undersea kingdom of Atlantis (and when I say "epic", I mean "epic". Namor's quest to regain his throne and defeat Krang lasted eighteen issues, practically a lifetime in an era where the average story was an issue long, tops.) It introduced all sorts of supporting characters, from his Grand Vizier to his lady love, Dorma, and most of his primary antagonists--Byrrah, Krang, and Attuma.
But the key change came when he finally graduated to his own series. There, he discovers that it wasn't nuclear testing that devastated Atlantis after all--it was an evil psychic named Destiny. (No, not the frail old lady who spent most of her free time writing books that Chris Claremont would use as plot devices years later. Different Destiny.) Why the big shift?
Because for all that Namor's tortured anti-hero schtick is integral to his character, he needs to be sympathetic to attract readers. And a hero constantly trying to destroy New York City and crush the hated Americans for their crimes against his people is, well, kind of a tough sell. (Imagine trying to sell "The Bombastic Bin Laden!" as a comic, and you'll get the idea pretty quick.) So he needed to be softened just that tiny little bit, much like years later, other hero/villains like Rogue and the White Queen would be softened in the same way. Still gritty enough to keep their edge, but not actively evil anymore.
And ever since, Namor's trajectory between "loveable jerk" and "outright villain" has pretty much followed an easily chartable path, depending on whether or not he has his own series. When he's a protagonist in a book that needs to sell, he loses his hatred of the surface world and becomes a good guy. When he's needed as a villain, suddenly the surface world must pay! (John Byrne even worked this into continuity in the 90s "Namor" series, explaining that he's got a bi-polar disorder due to his hybrid condition.) It's an interesting "dual role" for a character who is a reliable second-tier cast member in the Marvel Universe, providing him with versatility...perhaps at the expense of his ability to truly carry a book for a long run, but he makes up for it by giving writers on other series a fun, fan-favorite villain to bring in whenever they need one.