I don't think he realized something in particular. I think he' shocked because he was expecting Kurose-kun to do something to him, but he did not. I think he is shocked at himself for thinking like that
I also don't think he realized anything, but I dont think the shock came from him expecting kurose to do something. In fact, it might be quite the opposite. It's what kurose did that surprised shirotoni. Kurose blamed the causing of the scars on himself, he blamed himself for shirotonis problem. So I think that Shirotoni is not just confused, but also heavy hearted, because Kurose is now carrying self hate for Shirotonis problem.
could someone explain what shirotani realized at the end of ch 10, after Kurose-kun notices the scars on his hands? I do get that shirotani is starting to notice that he's changing.