Namor has a long history and at this point alot of it is spent being a hero. He just puts Atlantis before the surface world most the time and that leads to conflict. Lokis nature is to be evil and trys to do better for Thor. Namor is heroic but also very narcissistic and will kill the entire wolrd if it means saving Atlantis. They dont have to lean the evil way they can make him an enemy only when Wakanda or the F4 come in direct conflict with Wakanda. But I get what your saying. Namor has done some unforgivable shit just like Loki. Hes sided with Thanos or Doom and other villians. But usually hes disgusted by them and does it only to save Atlantis and what's left of his people. But then again some of the same writers that had him do some awfull shit had legit heroes in Tchalla and Black Bolt sneak attack and shank him. So Anyway they can make him an arrogant king but who's not evil. The MCU tweaks characters all the time. And like I said hes a straight hero alot of the time so it's not that big of a departure. Just ignore his less heroic qualities. Except for the part where hes the worlds biggest dick, keep that. It helps separate him from Aquaman.