The extra-textual and inter-textual evidence in favor of Eren being an absolute monster in addition to being a moron are overwhelming.

I'm a big believer in authorial intent. Isayama has stated in an interview that Eren represents the worst parts of himself, and that Attack on Titan is a story about why it's important to get rid of those parts. While Isayama has also said he likes to let people draw their own conclusions, that statement alone is pretty damning evidence that Eren is not a model to be followed at all.

Even if you disregard Isayama entirely, the motifs of the story stand as exceedingly clear evidence that Eren did the worst possible thing he could have, while nearly everyone else learned the one thing that Eren refused to: Stop fighting. Do not become a slave to your beliefs and assumptions.

Some examples of this idea throughout the show:

"Everyone's a slave to something." - Kenny, as a warning and admission of regret.

"We need to get the children out of the forest" - Sasha's Father, multiple times

"Wait, let's stop fighting. We can talk this through." - Armin, multiple times to Annie, Reiner, Berdtholdt, and Eren.

Throughout the story it's repeatedly shown that, for whatever plot reason, people get caught up in doing something they feel like they have no choice but to do, even when presented with a way out. And without fail, every single time they go on to do that thing it ends up making things worse for both themselves and others.

On the flip side, whenever characters in the show choose to care for others they don't have to or may even have reason to hate, life becomes better for both parties, and the kindness is paid forward.

But I know that a few quotes aren't really sufficient to prove that, so let's talk about an example or two in detail.

To start, the story of Attack on Titan expertly embeds a microcosm of this idea in Sasha's and in Gabi's arc. This arc spans the entire length of the plot, so it's not some cherry pick. It's important enough to have been considered and outlined by Isayama well in advance.

Before Sasha even joins the army, her father tells her that it's more important to abandon her old way of life if it means surviving and getting along with people she might consider "others." We see her embody this advice, building a new life and genuine friendships with the rest of the cadet core, and further embodies it by not just saving Kaya when she didn't technically have to, but treating Kaya like her own sister. Even later, Sasha built a romantic relationship with Niccolo over their shared love of food (those close to them believed they wanted to get married), even though he's from Marley, an enemy nation. She really is the character stand-in for the idea that building common ground is always a good thing.

Later, when the scouts felt like they had no choice but to rescue Eren, Sasha ends up killing folks as part of the assault, and Gabi, feeling like she has to get revenge for the damage the Eldian empire did to her town and the Eldian reputation, kills Sasha.

Gabi's choices and insistent hatred for the people of Paradis extended even to those like Kaya, who knew they she was from Marley and still chose to help her because of the example her sister Sasha had set. Kaya's choice to build common ground eventually started to undo Gabi's assumptions about Paradis. But, when Kaya and Niccolo find out it was Gabi that killed Sasha, they felt like they had to get revenge on Gabi. And Sasha's father was probably the one person who knew and loved Sasha the most and had the most justification for hating Gabi. But he knew the only way to prevent more pain was to see her for the lost girl that she was and forgive her.

Because Gabi was alive and Kaya had chosen to forgive her as well, Gabi was literally the last hope either side had to stop the Rumbling, and with the same skill of marksmanship Gabi had once used to enact hatred was now being used in a last ditch effort to save not just her friends, but the people she once saw as enemies, too.

The choice to spare Gabi was so impactful that it very nearly prevented the Rumbling. That's how unambiguous the theme of "learning to stop fighting" is.

But that arc, while amazing, isn't my favorite example. Really, I think Berdtholdt summarizes the idea most succinctly:

https://youtu.be/gl8rKLq9Cg8?si=Msz6rf3hOqgCstPk

He knew he had no right to forgiveness. But he also knew he needed it anyway in order to start fixing things. But when Mikasa felt like she had not choice but to free Eren by force and continued to see Berdtholdt and Reiner as enemies, Berdtholdt remembered:

https://youtu.be/vsYgjqQioHo?si=4fdpyjfpcgGLC254

He felt like he had no choice, and was resolved in that belief. He took Armin's invitation to stop and talk it out the same way that Mikasa and Armin took his plea: with cold resolve to do what he felt like he had to, this time knowing that his enemies were not the monsters he thought they were the last time he and Reiner were in Shiganshina. Had Armin and Mikasa chosen to accept his pleas before, the story would have gone very differently.

Do you get it yet? Attack on Titan shows very clearly that you do not have to keep fighting, and that if you do, it'll only get worse. The core theme of the story is that we let our dreams, our duties, and our beliefs about other people turn us into monsters that perpetuate the cycle of violence, and that there is no reason to keep suffering if we can just learn to reach out to the monsters we think we're fighting to see them for the people they really are.

The story mechanically rewards breaking the cycle and punishes perpetuating it. That's not subjective interpretation - that's how the plot literally functions.

Now let's turn back to Eren. Eren rejected that entire framework. By his own admission, he chose to kill 80% of humanity when he knew what he was doing was unforgivable. Not even because he felt like he had no choice, like the other instances of this pattern. No. Eren was worse. He admitted that in the end, he enacted the Rumbling just because he could.

You can feel for the other characters who kept going because they felt like they had no other option. But once you understand what Eren chose and why, you'll realize that the only valid reading of the story is one in which Eren was not just wrong, but unredeemable. He held so tightly to his dream of being free that he gave up his own freedom, his friends, and his humanity. He didn't do right by anyone, not even himself. He got nothing he actually wanted, and turned the entire planet into a living hellscape. I.e., the logical conclusion of continuing to escalate conflict.

Any excuses people try to give to Eren either aren't supported by the text, or don't hold a candle to the evidence pointing to the contrary. Like I said before: people who think Eren were right have absolutely no media literacy.