I think that statement could have been translated better: another way to have translated/interpreted 'Killing her was never the preferred option' was 'the option of killing her was never a priority/consideration". ((do we have another korean speaker to double check?)
This makes more sense because he was mostly indifferent to Ran for the most part. He was resentful that his father re-married, and this didn't like Ran or her mother very much, and especially not after he was beaten after he left Ran alone where she got lost.
Currrently, he's just intrigued and doesn't know how much he can trust her. This non-blood "sister" who he doesn't really talk to, suddenly says she's on his side. He knows everyone is out to steal the dukedom from him, and why is she any different? He's still suspicious of her, especially after she shows how capable she is in running the dukedom and how she got the ring spirit to accept her. After he learns she's scared of him, you can think of it like this: She's scared that he'll kill her... but he has no reason to. Why would he?? She promised to help him, but also she's afraid of him... why?? These words actually bother him a lot and they'll revisit this around chapter 20 again. (the anticipation! π)