Khan planned to execute the entire TOS bridge crew one by one in "Space Seed", by suffocation, while forcing the others to watch. How on Earth is he not a murderous monster? This wasn't a bluff, he was actually going through with it and thought Kirk dead.
FWIW, I'm pretty sure Spock's line in ID about Khan's plan to eradicate everyone not considered genetically superior was a based on Greg Cox
's Eugenics Wars
novels, where Khan's final solution was a modified version of strep which would have killed everyone on Earth who wasn't genetically engineered. His plan was foiled and Khan and his people exiled.