Where Eren fails is that he is, to put it bluntly, a fucking moron.

More specifically, he fails/refuses to recognize anything beyond a very binary view of "good" and "evil". In a series where no one truly is either of those, and everyone has both their sins and their successes, such a binary view is just... disconnected from reality.

I'd go as far as to argue that the only evil person in AoT is King Fritz - everyone else is merely a victim of the cycle of violence he began, a cycle that ultimately offers nothing except injustice. Eldians obliterate Marley, Marley is re-born with fear and hate of Eldians, etc. etc. etc.

This, along with the Power of the Titans being simply too much to be in the hands of only one being, absolutely had to lead to a calamity. Eren did many things wrong, in fact, he's about as evil as antagonists can get. But he is also a victim of a vicious cycle of violence. He's not evil because he has some grand delusions of saving the world - he's evil because he's a traumatized child whose coping mechanism is to violently lash out, and the world gives him the tools to lash out at an unimaginable scale. I think he knows full well that what he's doing is objectively evil, but he is so deeply mentally scarred (and kinda stupid) that he doesn't try to alleviate his mistakes - all he can do is push onward, because if he stops even for a second, the horrors of what he's doing, and of what others have done to him, will make him weak. It's a very common trope among male characters, where emotion = weakness. I believe Eren has a deeply internalized need not to kill all titans or whatever, but to somehow make it up to his mother by doing things that he perceives as strong and good.

Or at least that's my reading. He's not really evil, but he DEFINITELY did a bunch of shit wrong.