Re: Trek II ~ Khan's Wife
At first Khan merely used Marla as a tool to gain control of the Enterprise. But at the end of "Space Seed," he said he considered her a "superior woman." He certainly could have developed genuine feelings for her.
As for Khan slapping Marla around or physically abusing her -- what scene was that?
I agree. Khan's genetic enhancements resulted in his uncontrollable ambition and thirst for power, but there's no evidence that they prevented him from showing compassion to another being or from falling in love with another person. He seemed genuinely impressed by her courage in agreeing to stay with him on Ceti Alpha V. It's not difficult to believe that he learned to love her during the intervening years.
"He clapped his captain—his friend—on the shoulder. Yes, this man was very much like James Kirk, in all the ways that mattered." --Christopher L. Bennett-- Star Trek: Mere Anarachy, The Darkness Drops Again