He goes by many names. Prince of Atlantis. Lord of the Seven Seas. Avenging Son. Whatever name you use for Namor McKenzie, the only consistent factor is that he is powerful and has done some truly terrible things with that power, both as a hero and as a villain. He has fought the Avengers, the X-Men, and the Fantastic Four almost as many times as he has stood beside them in battle. In truth, everything he does is for the betterment of Namor first and foremost. Everything and everyone else comes second, even his kingdom of Atlantis.
10 His Pursuit of the Invisible Woman
Though their relationship hasn't always been a smooth one, Mister Fantastic and the Invisible Woman have been referred to as the first couple of the Marvel Universe on many occasions. That reputation certainly didn't stop Namor from trying to woo the Invisible Woman. Admittedly, Namor's good looks and charisma did catch Sue Storm's eye but she did, more than a few times, refuse his advances. Namor was undaunted and has continued his pursuit, causing no end of tension between him and the rest of the Fantastic Four. Fighting alongside someone while trying to steal his wife is not cool at all.
9 Turning His Back on Professor X
Post World War II, Namor found himself afflicted with amnesia and travelling around America on his own. He was eventually found by Charles Xavier, who informed him of his status as a mutant, given his hybrid Human/Atlantean physiology. Namor agreed to go with Xavier on a journey to find more mutants and help them.
The first mutant they found was surrounded by police and on the verge of being killed. Xavier restrained them with his powers, but Namor argued with him about what to do next, causing Charles to lose control. The mutant was then killed and Namor tried to kill the police in retaliation, only to be stopped by Xavier. Namor once again went out on his own.
8 Gamma Level Team Up
Like Namor, the Hulk finds himself frequently straddling the line between hero and villain. In the early days of the Avengers, Hulk and Namor loosely teamed up to fight them, taking the young superhero team to its limit. Not surprisingly, the Hulk abandoned the battle when he got tired of fighting the Avengers, leaving Namor on his own and forced to withdraw. Namor did not take this well. After the fight, the Sub-Mariner stumbled across an Inuit tribe worshiping an ice deity, which Namor took and tossed in the ocean. The ice deity turned out to be a frozen Captain America and Namor's actions led to him being freed. This was one time him being a jerk ended up doing something good.
7 Siding with Doom
On more than one occasion, Namor has found himself standing side-by-side with Doctor Doom against the likes of the Fantastic Four. Much like Namor, Doom is leader of a nation and an individual of great power. Strangely enough, Doom is one of the more consistent friends Namor has, as much as Doom is anyone's friend.
Doom was the only leader or hero in the surface world to step up and offer Namor support after Kamar, Namor's long lost son, attempted to overthrow his father. Doom was also there for Namor when his subjects were forced to abandon Atlantis. That favor led to Namor joining with one of the greatest super villain teams in Marvel history, the Cabal.
6 Target: Wakanda
Wakanda and Atlantis were on the verge of a massive war at one point, but Namor and T'Challa, members of the Illuminati, were able to negotiate a peace treaty while dealing with incursions by other dimensions. Unfortunately, Shuri didn't agree and prepared the forces of Wakanda for an attack on Atlantis. When Namor returned home, he found Atlantis in ruins and Proxima Midnight ready to attack in search of an Infinity Gem. Namor used the moment to trick Midnight into attacking Wakanda. This action caused Proxima Midnight and the rest of Thanos' Black Order to find some incredibly destructive weapons belonging to the Illuminati as well as inflicting heavy destruction.
5 Secret Empire Tyrant
While Hydra and their evil Captain America ruled the surface world, Namor tried to steer clear of the whole situation and protect Atlantis. His idea was sound but his execution was terrible. The King of Atlantis went full tyrant, controlling everything his people did and crushing anyone who opposed him.
This even led to the attempted public execution of his own cousin, Namora, which was stopped at the last minute. He eventually came to his senses, in no small part due to the hundreds of Atlanteans attacking him, and started acting like a proper ruler and hero, again.
4 Unleashing Giganto
Once again suffering from amnesia after his encounter with Professor X, Namor was left as a derelict for many years. Johnny Storm saw him use his powers and helped him regain his memory. Now armed with full knowledge of who he was, Namor tried to return to Atlantis but found the wreckage of an outpost instead. He erroneously believed Atlantis had been destroyed by humans and summoned the immense sea monster Giganto to bring his vengeance on the surface world. It was only through the efforts of the Fantastic Four that Giganto was repelled and the surface world saved from Namor's rage.
3 Breaking Randall Peterson's Heart
Three generations of the Peterson family played a large part in Namor's life on the surface world, especially Randall Peterson, an honorary member of Namor's World War II team the Invaders.
While trying to keep Namor from starting an all out war with the surface, Captain America and Jim Hammond met with Randall. Namor found out, showed up in an absolute rage and launched Captain America into the ocean. Disappointed in his friend's actions, Randall yelled at him, the stress of which caused his heart to give out. Had Namor kept it together, Randall would have been okay.
2 Destroying Wakanda
Alongside four other mutants, Namor received a portion of the Phoenix Force and became one of its avatars. They used their powers to build a massive Utopia for all mutants as well as hunting down anyone who opposed them, like the Avengers. Seeking to finish them off, the powered up Namor tracked the remaining Avengers to Wakanda where he assaulted them with massive wave of water. This attack was brutal and indiscriminate, killing scores of innocent civilians and causing incredible damage to large portions of the African country. He was defeated by the Avengers and lost his Phoenix Force powers as a result.
1 Destroying Earth Over and Over
As the incursions became more frequent and dangerous for Earth, the Illuminati disagreed on how to handle the situation, with Namor on the side of destroying the invading versions of Earth to protect his own. Despite the protests of his own team, Namor triggered a device that destroyed a universe entering their reality via the most recent incursion. He essentially committed mass genocide, something that did not sit well with the Illuminati. The move did sit well with the new Cabal, which included Thanos. This new team continued to destroy universe after universe, presumably to save their own reality and not just because many members of the Cabal enjoyed it.