&gt; him doing it for his friends has always been part of his character

I'd argue a little bit about that. It's true Eren cared about his friends. But before he discovered what is outside the island, his main motivations were revenge for his mothers death on the Armored Titan/Colossal Titan (Which he personally blamed for) and finding out what's beyond the walls. After finding out the truth his main motivation was to defend Paradis at all cost. That includes not only his friends, but each and every one of its citizens. His motivation remained absolutely consistent for 123 chapters and dare I say - 138 chapters (despite the storywriting blunders post-123). Post-timeskip he was willing to sacrifice said friends, and even Mikasa, for a greater cause (the attack on Marley, for example). At no point, even in his thoughts, is it ever hinted at that he is sacrificing his life for his friends to become heroes (and that in itself is an absurd motivation because in the world's eyes Paradis just genocided 80% of the world population.  Hell no, people do not think like that).

 Lelouch makes sense because he was always driven by the same goal - creating a better world for his sister and he wasn't sure how to do it, so most of the story revolves around him trying to find a way and he inevitably comes to the final conclusion. Lelouch also never did something as nasty as Eren did and did everything possible to distance himself from his country while Eren associated his nation and his friends to initiate the attacks.

Hell, Dany from GOT made more sense because her turning evil was actually set up, it was just incredibly rushed and done in a very bad manner and needed a couple seasons of growth in that direction. 

Eren should have never Lelouch-ed himself. It should have always been a story about a tragic figure doing something horrible while realizing what he is doing is horrible, but not finding any way out of it. And in the end the stakes should have been simply higher with most of the cast dying and leaving a bittersweet taste in the mouth of the viewer. Instead... practically every on-screen character survived for some reason.  Call me a conspriator, but I am of the belief that the editors at the magazine basically forced him to do something more shounen-like. And it's unfortunate because this manga truly had the ability to revolutionalize the genre, but instead it turned into another generic shounen that you do not want to read ever again because all the great moments have been utterly ruined.

Anyway, a long rant, I know. But this ending is straight up bad and I'm amazed that some people on other subs are actually defending it.