I would like to point out for a second that if Ishida does indeed decide to kill her off for good, then my respect for will reach a complete new point. If you really think about it, doing something like this not only does it take huge guts but also it is very realistic if you ask me and that's why so frightening the possibility of it coming true. We read stories expecting to draw conclusions about the characters based on their actions and personalities, yet in reality, not such chances are given necessarily. Eto is important for us, the readers and therefore for the plot but she's not important for Kaneki. She's the cause of many tragedies, including indirectly his own. He doesn't care about her, he doesn't want to understand her, he just wants to get rid of her. In a sense this comes in a direct contradiction to what "tragedy" stands for which is catharsis, however this catharsis only happens in literature, tragedy in real life has nothing do with catharsis. In reality, people die without given any chance to fix their mistakes, anonymously, and we simply don't care for these people because we simply don't know them. That in a sense is the meaning of the true tragedy and this right here is a reminder of what reality looks like.
