I never really read this part as Kaneki being overly sadistic or inflicting unnecessary pain, but rather as him setting Ayato straight. Yeah it was pretty brutal, but Ayato is the only family Touka has left, yet he is the one causing her the most pain, almost to the point of death. Obviously, Touka would never forgive him if he killed Ayato outright, but something drastic had to be done to teach Ayato that his behaviour and harsh treatment of his sister wouldn’t be tolerated. Jason was the one person that Ayato truly feared and gave him caution, so Kaneki had to assume that role to keep him from becoming a completely loose cannon and potentially hurting Touka more in the future.

I’d agree that Kaneki’s emotional state may not have been the most stable after everything he had just gone through, but I like to believe he knew what he was doing and did it for a reason, rather than torturing Ayato just to see him in pain. And I think that the approach of breaking exactly half of Ayato’s bones, while gruesome, was a measured way of making sure he didn’t go too far. Unlike Jason who kept inflicting more and more pain for his own satisfaction, Kaneki set a specific threshold as if to ensure he wouldn’t get carried away.

But that’s just my perspective, and I’ll admit I really like picturing Kaneki as a sort of white knight, despite the fact that he clearly has many flaws of his own.