Nobody's talking that killbill horseshit. It's simple, technical fact. Superman cannot be something he wasn't born to. He was born on Krypton, he is kryptonian. Logically, that means he cannot be an american because he's not even an earthling. People from the US are americans, just as people from germany are germans, brazillians from brazil, spanish from spain, chinese from china, ext. Citizenship is something that can change, as that is determined by where you live. Nationality is determined by where you're born, as you cannot change your place of bith.
Therefore, Superman, while he is a US citizen, is not, and cannot ever be, an american because he was not born on in the United States of America, nor was he born on Earth.