Nagare was special to Okaya.
I originally thought with the themes of the story Okaya would have family issues or some family trauma that made him just want to mess with Nagare for having such a good family life. I think Okaya has some abandonment issues or people have lost their resolve with him and fighting him before so something clicked with Nagare fighting and being so determined he was probably left behind for being awful or someone became despondent with his behaviour and bullying and he lost hope and thoight everyone would give up eventually but Nagare was strong in a specific way that had Okaya hooked. I don't believe Okaya wanted to kill Nagare I think he wanted him around for a long time so he was disappointed not grieving his death.
On a side note: Toxic doomed yaoi moment.
That guy was definitely obsessed with him. Which probably lead to the worse bullying.
I still don't have a read on him, but that guy is not fucking sane