Though he (Erin) does become a complete, insufferable emo prick by the final arcs of the series
I don't think Eren's demeanor is described well as "emo" or anything of the like. His entire worldview changed when he realized that beyond the walls, not only had all the land already been explored and claimed, but there were billions of people who were actively
against the freedom of Eldians/Paridisians. Instead of a free and undiscovered world he had always dreamed of, he was met with this cruel reality. But that's not all that changed him.
Eren was aware that he would commit mass genocide from the end of season 3 onward. When he kissed Historia's hand, he saw past memories, yes; but he also saw future memories of him activating the rumbling and killing innocents with his army of wall titans. Seeing something like that changed his personality drastically. He had to grapple with the fact that
he himself would apparently commit mass genocide in the future.
And yes, Eren did attempt to look for other ways out during the four year timeskip. But he eventually resigned himself to wiping out the rest of Earth's population in order to protect the people he cared about, and accepted the role of a devil he would be given as a result of his actions. Eren completely hates himself after the timeskip. How could someone not hate themselves if they fully recognized the gravity of their actions, those actions being the massacre of billions of innocent people?
tld;dr I think that Eren's development is really well-written.
Also, I welcome various perspectives on works of fiction, so please don't take this as a rant lol, as it wasn't intended to be. It's just my own understanding. And I'm pretty into SnK, so I try to understand as much as I can about the story and its characters.
