To take things out of order: He's only in the position of relying on Marla's loyalties because his automatic reaction to the world around him is an attempt at conquest. (I need me one of them nifty holiday-season nicks...) Except... we're told quite explicitly that they don't in both the words and actions of the Enterprise crew. They represent dictatorship, they represent science gone wrong, Khan certainly represents overweening arrogance... but they also represent vast potential. In the quote you provide, Khan isn't just some deluded racist twit who mistakenly imagines himself to be superior, he actually is measurably superior -- a character conceived as a giant of a man. Just not in the moral sense. That's why they are the "Space Seed." It's why they're seeded on a planet at the end of the episode in order to give them more time to adjust to, or develop into a condition to join, the modern age. Of course all of that is also largely absent from STID Khan, that's another difference in the writing.