Eren's character transformation has to be the craziest an anime/manga character has ever undergone. nikka used to be quick tempered, making speeches and thought he was change the world was waiting on like like a typical shonen protagonist but learned over and over how weak he was and how little he knew, then he got to the point of believing his father killed Freida and ruined the chance for titans to be wiped out only to learn that his father begged for Freida to eliminate the titans before he stole the founder and killed her and her siblings. Then he found out the evil mysterious titans that he's wanted to kill were all human beings once that belong to a persecuted race who were forced to become titans and eat their own people. He's been seeing previous Attack titans memories of the world and knows all too well shyt is complicated and easy, no talk ninjitsu will fix it he just gotta do what he gotta do by any means.