Misery Loves Company
As he talked with Roody and Ken, Eren realized that his anger at Ken was misplaced.
Maybe one of the reasons he hated Ken was because of the latter's ability to nurture goodness in his heart even after coming to terms with the cruelty of the world. Ken maintained his fairness-and-justice-seeking path despite knowing the potential repercussions.
No matter how he tried or pretended to, the butcher wasn't capable of nurturing goodness in his heart anymore. For better or worse, he had aligned himself with the cruel side of the world. So in the past, to make sense of his inability to nurture goodness, he branded Ken as his opposition.
Eren could say that he was still in his growing phase as well. Because it took him this long to realize how naive he had been in his academic days. This is despite being the only student there at the time with the mindset of an old man.