Perhaps he felt a responsibility to speak out and prevent others from making similar mistakes. Sharing his story could also be a way to seek support and understanding from the community.
In the manga, Kaneki's experiences and circumstances lead him to make choices and exhibit behaviors that some might consider evil. It's a complex development throughout the story.
In the manga, Ayato survives his encounters with Kaneki. The story doesn't have Ayato being killed by Kaneki. Their interactions are part of the complex plot but not resulting in Ayato's death.