He's been allied with Dr. Doom in the past, as well as many other questionable characters. But none of them to my knowledge have actually destroyed Atlantis and killed him like the Squadron Supreme has. I don't see it happening unless there's some form of extreme blackmail involved.
And I'm guessing he bothers with the Atlanteans because he's their King or something. Being half Atlantean with a high sense of duty also probably has something to do with it