Having seen Taxi Driver last night, I woke up with a gripe against the idea of an evil man who is just evil, and once again unhappy about Khan's treatment.
Space Seed is overflowing with examples of Khan's evilness and ruthlessness, and one can cite them to no end, but it did go out of its way to add ambiguity to his character. It was everywhere, it was in the undertones, it was in the crew's impression of him from history books. And they did leave him on Ceti Alpha V optimistic and intrigued about the future of the colony – which shows that even the crew of the Enterprise didn't feel he and his crew were irredeemably evil.
And we're in the Star Trek universe, where any kind of being was able to become part of the family, regardless of their strength, intellect, or values, so it is regrettable that, outside of DS9, any genetically-enhanced person was deemed unworthy to be a part of the civilisation. I would imagine it would have found a way for better inclusion of people with chronic lack of empathy for others, which could have worked even for Khan. TWOK and STID threw out any ambiguity and wasted his character potential to make him more than the token evil guy (I saw TWOK before Space Seed, so saying it ruined Khan for me would be a very peculiar statement I couldn't make... so I'd make it
STID even played into it, by bringing his pain, by making him mistreated by Marcus, and by all that, and in the end it went nowhere. Thankfully, he's alive and well, so I guess this opportunity isn't completely lost, but chances are, he'll be as evil if he returns. We all know that. (Besides, adding anything positive to him now, after he murdered all those people in SF would be a very challenging task.)
Then again, I was generally underwhelmed by the final sequence. For example, I would have liked if McCoy broke the news of what happened in SF to Kirk when he woke up in his bed – with his fitting emotional reaction. So maybe it is a lot of small things that make me generally biased against everything in it, including Khan's fate, which did make sense.
As for the genocide line, while seemingly unfitting, I do think that "ethnic cleansing" is an unfortunate term that began as euphemism to belittle what was going on, so I wouldn't be surprised if in the future it is replaced with "genocide" even when it doesn't involve literally killing people – maybe Khan didn't commit that kind of atrocity, but he chased everyone who he found less than superior in ways that would have ultimately lead to their demise – perhaps killed by the other tyrants. So it is quite possible that he committed no massacres, but was committing mass genocide by sending people to placed where someone else would commit a massacre, or where they would have little chance of surviving. Wait, isn't that already genocide?
Besides, he could have just forbidden these people from having children. That is genocide under the current definition, and it is not a massacre. Does Space Seed say anything about Khan allowing everyone to have children under his rule? No? Well, Spock's statement does not contradict anything then.
And that makes him a Hitler too.