Unless Arima said, "I'm your father".
I don't really understand why Kaneki should have any feels for him. Stockholm syndrome much? I have to re-read, I don't really understand the changes that have gone inside Kaneki's mind the last 30 chapters. Reverting back to White-Haired-Kaneki, symbolism, and all. Refusing to finish off his enemy, like with Jason, but for other reasons. He respects Arima, contrary to how he viewed Jason, but what happened to his resolve to fight for his friends' sake? His desire to live... Killing Arima is the safest path. Arima killed himself, but what would have happened if Kaneki let Arima live this time around?
He should have just walked away in surprise. I like that SIU threw that change-up with suicide, but Kaneki shouldn't feel anything for Arima. Arima was his torturer, captor, tormenter, etc. I know there's a reason behind Arima investing in Kaneki, but we still don't know if it was for a bigger cause or his self-desired death.
Btw, go save Eto :/
Need more time to re-read :/