Of course he was using her as a means to an end in that setting (being held captive and sent a slave woman to spend the night with). I assume when you say he wasn't "just" using her you mean he enjoyed it as well, thought she was beautiful, had compassion for her etc.. This really doesn't negate the using part. It's kind of repulsive because she's a sex slave, not just a beautiful girl who he sleeps with mutually to further a mission. She's in his room because she was ordered to be there and she cannot say no.