Ken Kaneki was horribly tortured and assaulted by Yamori over and over again for days. During the torture Kaneki, he went through so much pain and had hallucinations. In one of the hallucinations, he had a conversation with Rize where he told about his past and about how wonderful his mother who overworked herself to death. Also, in order to break Kaneki, Yamori brought Shuu and Haru in front of him and demanded him to choose who should be spared. Since Kaneki was unable to choose, Yamori killed both of them. This filled Kaneki with more guilt and he realized that he was weak all along and lived by the belief of 'letting himself get hurt rather than others', which is the reason why his mother suffered so much. Rize warned him in his subconscious that if he did not sacrifice one thing for the other, his loved ones would die due to his incompetence. He was emotionally traumatized. Finally, he ate Rize in his subconscious and accepted himself as a ghoul and went through a transformation. The more he was hurt, the stronger he had become. He no longer was driven by human emotions. He became ruthless. He finally defeated Yamori, tortured him the same way he was tortured and ate his kagune to harness more strength.
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