The Sub-Mariner Namor is one of Marvel's oldest and most storied characters. He's also flipped sides more than Magneto and Doctor Doom combined. He's worked with the Avengers, the Fantastic Four, the Invaders, the X-Men, and even owned an incarnation of the Heroes for Hire. He's also battled almost all of these teams, frequently starred in a book called Super-Villain Team-Up with Doctor Doom, and led the Cabal, among whose number were Maximus the Mad, Terrax, and Thanos.
Needless to say, Namor has screwed over both his hero and villain allies many times. Sometimes, there can be an argument made in his favor, but that doesn't make the betrayal any less rough for those he's left in the lurch.
10 Shutting Down The Heroes For Hire
Firstly, Namor pulled the plug on Oracle Inc.'s Heroes for Hire. For the uninitiated, Oracle Inc. is the company that Namor started as a front for Atlantean interests as well as a means of cleaning up the ecosystem in John Byrne's 1990's Namor comic. Later, Oracle would enlist its own Heroes for Hire team under Jim Hammond, aka the original Human Torch. After many trials and tribulations, Namor returned to announce that he'd sold Oracle Inc. to Stark-Fukijawa, effectively putting the Heroes for Hire and Jim Hammond out of a job (and ending the comic series).
9 Letting The Shroud Try To Kill Doctor Doom
During the aforementioned Super-Villain Team-Up series, Namor often found himself working alongside Doctor Doom. The two worked together and helped one another achieve their goals. They developed a mutual respect and even saved one another's lives.
Of course, both Namor and Victor are still volatile egomaniacs, and it was inevitable that they would find a reason to turn on one another. After one such instance, Namor sat back while the Shroud attempted to kill Doctor Doom in his own country. Whether or not Doom deserved such a fate, it remains true that Namor turned on his close ally and let someone try to feed him to his own dogs.
8 Leaving The Cabal To Die In An Incursion
Speaking of leaving people to die who arguably deserved it, Namor tried to abandon his Cabal to die in an Incursion. Namor's Cabal, which consisted of himself, Thanos, Maximus the Mad, Terrax, Black Swan, Corvus Glaive, and Proxima Midnight, were traveling from Earth-to-Earth in an attempt to destroy the other Earth before they could collide. Eventually, Namor decided that the gleeful slaughter carried out by the Cabal could no longer be tolerated and coordinated with the Avengers to leave the Cabal on a dead Earth before detonating it. The follow-through didn't quite turn out how Namor hoped, though. As Black Panther stabbed Namor and had Black Bolt blast the Sub-Mariner back to the dead Earth, intending for Namor to die when the Earth was detonated. The Cabal and Namor narrowly escaped to the Ulitmates Earth before the dead Earth was destroyed.
Like Doom, the Cabal probably deserved such a fate, but Namor brought them together and led them in the first place.
7 Trying To Steal Invisible Woman From Mister Fantastic
Little love has been lost between Mister Fantastic and Namor over the years, but the two men have helped one another often and formed a mutual respect - sort of. Despite the many times that Reed and Namor have stood together, this hasn't stopped Namor from trying to win over Reed's wife, Susan Storm, aka the Invisible Woman. This is a trend that continues to this day, no matter how many times Sue shows she is devoted to Reed.
6 Flooding Wakanda
When the Avengers and X-Men went to war, Namor sided with the X-Men. In the midst of AvX, Namor, Cyclops, Emma Frost, Colossus, and Magik each possessed a piece of the Phoenix Force. This granted each incredible power and forced the Avengers to go on the run. Earth's Mightiest eventually fled to Wakanda, and Namor used his incredible power to flood Wakanda with a great deluge - killing countless people.
This led to a war between Wakanda and Atlantis, which ended in both being virtually wiped out and Black Panther leaving Namor to die in the aforementioned Incursion. Secret Wars (2015) and All-New, All-Different Marvel seemed to (mostly) reset Wakanda, Atlantis, and the animosity between T'Challa and Namor.
5 Forming The Cabal
If leaving the Cabal to die was a betrayal, forming it in the first place was a greater one. Before Namor brought this group of killers together, he was a part of the Illuminati, a union of Marvel's greatest superhuman minds and leaders consisting of Mister Fantastic, Iron Man, Beast, Black Bolt, Doctor Strange, Black Panther, and Namor. Their intent was to stop the Incursions from wiping out the multiverse, but they splintered after all but Namor found themselves unable to wipe out another Earth to save their own. Namor's destruction of that other Earth, and the slaying of the Justice Leagueesque heroes that inhabited it, led to another fight between T'Challa and Namor and ended with Namor leaving the Illuminati. Shortly after, he formed the Cabal, and they slew countless Earth's in their bloody mission to save the 616 Marvel Universe.
4 Allowing Doctor Strange To Wipe Captain America's Memory
Also, Captain America was briefly a part of the Illuminati and used the Infinity Gauntlet to halt an Incursion and save both Earths, which led to the Gauntlet and its Infinity Gems to be destroyed. After this, the Illuminati began discussing the possibility of destroying other Earths should another Incursion occur. Steve Rogers, one of Namor's oldest friends, wanted to put a halt to this discussion immediately. He wouldn't make that tradeoff, but the rest of the Illuminati were intent on keeping this option on the table. Doctor Strange used his magic to KO and mind-wipe Captain America while Namor stood by and watched.
3 Almost Killing Stingray
Namor has been on a new warpath of late, and it started with him attacking Stingray. Stingray, aka Walter Newell, is a Marvel hero who wears a powerful armor suit, which grants him the strength and maneuverability to survive the depths of the ocean. He can fly, shoot lasers, and has super strength, but he mainly works underwater and was good friends with Namor. Stingray aided Namor in fighting Tiger Shark and finding Leonard Mackenzie, Namor's human father.
This friendship was violently disrupted when Namor decided that he was going to war with the surface, and he found Stingray helping a sinking ship. Namor unleashed sharks to maul Stingray, and Walter barely survived.
2 Cutting Off The Torch's Head
Captain America, Namor, and the synthezoid Human Torch fought together against the Axis Powers during World War II, facing death countless times. This didn't stop the Sub-Mariner from decapitating the Human Torch during his current campaign of war against the surface world. Captain America, the Winter Soldier, and the Torch were doing whatever they could to peacefully defuse the tension between Atlantis and the United States. During a battle between Atlantis and Namor in a coastal town, Namor cut off the Torch's head.
However, Hammond is an artificial life form, and Tony Stark was able to rebuild him with a new and improved body. He lived to become the Iron Torch and continue his fight with Namor, but that doesn't change the cruel betrayal in Namor's action.
1 Going To War With The Invaders
Of course, Namor's mutilation of Jim Hammond is just a part of a greater sin: his war with the Invaders. In the most recent run of Invaders by Chip Zdarsky, Carlo Magno, and Butch Guice, the entire series was devoted to the Invaders trying to talk reason into Namor and stop him from going to all-out war with the surface world. Namor fights brutally against the Invaders in almost every encounter, and he even changes an entire coastal town to Atlanteans against their will. In the end, the Invaders are able to stop Namor's warpath by curing him of a psychic blight left by Professor Xavier decades earlier...but Namor's grudge against the surface lives on in a later issue of Avengers.