Alright, I'm seeing a few people express how they feel about what appears to be a romantic/sexual undercurrent to Okaya's treatment of Nagare. As someone with - unfortunately, a lot of experience with bullying dynamics, I figure I could shed some light on a different perspective. TL;DR: My bet is that it's infatuation, not love. Thought process below.
A weirdly prevalent phenomenon I and a few of my peers have noticed over the years in sadistic bullies, is that they do tend to find a favourite, however, their most frequent victim is rarely who outsiders-looking-in would deem the most "fun" to bully.
It's often victims who - in spite of options to reduce the trauma (Nagare running away as Okaya suggested), display resilience. Not so much defiance, though that has appeared also. More importantly, it's the idea that the victim perpetuates the dynamic regardless of the reasons for doing so. I've seen particularly twisted individuals say that their favourite victim is the one they spend the most time around, interact with the most, and so feel a warped sense of acceptance from them. Most victims will either run, break, or fight back. A victim who stays, in their eyes, accepts the dynamic. It's not love these bullies feel for them, it's infatuation, which will often manifest itself in similar ways.
In Okaya's case, it could very well be love; I have to say that I'm not certain just yet. Though if I had to bet on it, I'm going with infatuation for this one detail in Okaya's bullying: Okaya has never demonstrated any sexual deviancy in his bullying, but he has explicitly escalated his bullying simply to see how far Nagare would go, as if he's pushing to see how much he can do and still be accepted.