I have mixed feelings about it. But I think the moral ambiguity of the ending simply makes sense. From a certain standpoint, I can see why Eren took the actions he did. In the end, to preserve his people from the unjust prejudice of the entire world (though I wouldn't say Marley's fear was totally unfounded) he was willing to sacrifice many innocent lives, but he also made tremendous sacrifices himself, hinting to Armin that he manipulated time to send that titan to eat his mother, thereby ensuring that this version of events would come to pass. He lay down his life and the lives of those who most mattered to him in order to protect his people and their land. In the process he stained his hands with blood.
Eren never wanted to harm anyone, but global powers forced his hand. Once Tybur had made his speech, casting the Eldians of Paradis as a threat to world peace that must be destroyed to atone for historical crimes, Eren had no choice but to go forward with his plan to trigger the Rumbling in order to protect his people. The calamity that followed is on them, not him. Eren knew they could not let go of their hatred because he couldn't entirely either. It was all he could do to save his land and those dear to him.
His actions, while horrifying, achieved two important goals. He decimated the Anti-Eldian global alliance that threatened outright to destroy his entire race, weakening them to the point where no retalliation will be possible for many decades, and he erased the Titan power from the world, thus ending the accusation that his people were "devils", at least on the basis of the historical threat they once represented.
I think Isayama is simply a realist when it comes to these things. War will always be part and parcel of human nature. The rise of the Jaegerists is simply the inevitable consequence of vilifying an entire people on the basis of race and accusations of historical guilt. Nationalism is on the rise in many countries as a direct response to the Anti-White prejudice promoted by media and government. People mirror hostility and collectivise when threatened. It's sad but justifiable when the alternative is subjugation or death.
Individuals are forced to tribalise when others refuse to see them as anything but a single amorphous entity. By vilifying the Eldians and in particular, the people of Paradis, Marley created a mirror image of themselves in the form of the Jaegerists, who have now gained enormous power based upon the same accusation of historical guilt once used to justify genocide against their enemies. Eren's actions won't ensure the survival of his people, or bring an end to war, but the Titans at least are gone forever, his childhood oath was fulfilled, and he is, along with his people, finally free, as symbolised by the bird in the last panel.
From a critical standpoint, I felt the ending was a bit rushed, and I would have liked more of a resolution for the other characters, but overall it was better than I expected. I'm just glad we didn't get this -
