Space Seed, yeah I can see him being a genocidal supremacist cause of the open contempt he does display for normal humans.
Captain, although your abilities intrigue me, you are quite honestly inferior. Mentally, physically. In fact, I am surprised how little improvement there has been in human evolution. Oh, there has been technical advancement, but, how little man himself has changed. Yes, it appears we will do well in your century,
So the whole genocidal war criminal is fine by me
That doesn't make any sense considering he married a person who wasn't as superior as he was, but also a person who betrayed him. And do you really want to stick to that one time Khan talks about how superior he is? Because once Khan and his people capture the Enterprise and hold the entire crew captive, he continues this "superior" speech.
Khan: Nothing ever changes, except man. Your technical accomplishments? Improve a mechanical device and you may double productivity. But improve man and you gain a thousand fold. I am such a man. Join me. I'll treat you well. I need your training to operate a vessel this complex.
Join me and I'll treat you well. That does not sound like a man who wants to kill every last one of the crew without hesitation. These are his lines in regards to his time on Earth.
Khan: We offered the world ORDER!
Khan: On Earth, 200 years ago, I was a prince with power over millions!
Where's the genocide? Where's the "Everything that is below me must die!" speeches? There isn't any because that is not in Khan's character.