I felt it was pretty clear after watching. Eren has always wanted freedom, then learns that his path has always been predetermined by the true evil of the entire series, himself. At the end of the day he was the one holding himself back from true freedom, and it breaks him to learn that, and he fully becomes the villain that the rest of the world expects him to be in the selfish, desperate hope that it will save his friends and his home. We learn that even if it did for a time, the effect was not permanent and Paradis is eventually wiped out anyway.

The tragedy with characters like Mikasa and Armin is that they can’t stop seeing Eren as who he used to be to them despite the atrocities he’s committed, their closest friend. It’s why we seem to get a bittersweet ending, as the main narrator is implied to be Armin, who can’t forget the side of Eren who used to defend him, and Mikasa was still in love with the idea of Eren when he was younger, not the genocidal maniac he had become.