As I'm sure you're aware, that's a retcon based on the Augments arc of Star Trek: Enterprise, written and produced 37 years later.Augments like Khan, as we have seen many times, are incapable of genuine feelings like love or compassion - such things are literally bred right out of them.
As originally written, Khan and his followers were simply the products of selective breeding, not advanced genetic engineering. There was no suggestion that they were incapable of love or compassion -- only, as Spock pointed out, that "superior ability breeds superior ambition."
Retconned maybe, but not by STE. Chekov, in TWOK, says: "Criminal, Captain, a product of 20th century GENETIC ENGINEERING"