Well acroding to Isayama he did changed the ending to an extant but he's been said that first in 2017( when the post-time skip started in the manga acually) and he claimed that the changed happened back in 2014 after the anime came out and blew up. He said he was insprised to change it to a more happy ending for the genral fanbase. So the ending we got is the " happier one" to him make of that what you will. In later interviews he says he never really changed the ending but just the tone and how it was protrayed( some people don't belive him ).It is hard to figure out what went wrong with Attack on Titan. Part of me feels that Isayama became very stressed and jaded during the writing of the series and had some sort of burnout. The story was always grim and violent from the start but there was an underlying current of optimism that Eren and his friends were going to break through the overwhelming odds and try to create a better world. The idea that courage, honor, and friendship still mattered even in the face of terrifying monstrosities, government conspiracies, and the terror of being eaten alive. Where this started to sour is up for debate. I would argue it was in the aftermath of Historia/Krista rejecting her father's desire to have her inherit the Founder's Will and save Eren. That was easily the high mark of the optimistic outlook in the series. Even the period right after that that leads up to Historia's coronation is still tinged with hope, with Eren resolving to find a way to prevent Historia from being made a tool and saving his friends.
It is the 'Return to Shiganshina' arc where things start to take a swing back to favoring pessimism due to the immense losses among the Survey Corps, but if there is one moment that foreshadowed that this story was not going to have a happy ending it was when Eren, Mikasa, and Armin reach the ocean after clearing Paradis of titans. It is when Eren starts crying after realizing that instead of finding a world of freedom like he always imagined, that there were only more enemies awaiting his people. When he wonders if killing all of those enemies would allow for the Eldians to be free, it sets up the concept of the Rumbling.
Regardless, it is not as if the story had to go in that direction. Isayama could have taken the story in a lighter direction that would have shown, no the Eldians did not have to kill the rest of the world to live in freedom and peace. However, he set up the rest of humanity and above all the Marleyans as hyper-racist and genocidal warmongers who are hellbent on killing the Eldians of Paradis and will not compromise on their position, and then fight each other to control the world. Even token moderates like Magrath and Onyankopon (and Armin if we want to be honest) have no effect on this narrative because because they are totally ineffectual and their ideas never seem to be able to convince other characters to act better. You also have the absurdity of the Tyber family, who are Eldians who turn out to be the true rulers of Marley... and they want all Eldians dead too. Willy Tyber even sets up his own death so that the rest of humanity can be goaded to implement the Final Solution toward Paradis and other Eldians in the ghettos, his only mistake being that Eren was on to it and used the plot to further his own ambition of killing humanity first.
And that is not getting into the issues with time travel, alternate realities, and the revelation that Titans were born out of a girl being fused with a prehistoric life-form that somehow gave her reality bending powers... and she ends up in thrall to the barbarian chieftain who murdered her family and cut out her tongue and helped him conquer much of the known world and her soul continued to love him from her pocket dimension for 2000 years despite her being fully aware he only saw her as an object. What impresses me so much about how Attack on Titan fell apart is not how bleak it got, but how convoluted the plot had to become to allow that bleak outcome. Isayama introduced a whole slue of Sci-fi tropes that had no real place in the story to turn it into something almost unrecognizable from its original concept of low-tech humans fight man-eating giants; and I got the strange feeling when he had Eren say, "I don't know why I did it" it was an admission from the author how he had lost control of the story.
I personally do not regret being in on this ride for the past decade, but I would not revisit it again except to explore some of the crazier themes introduced towards the end. Attack on Titan is just too inconsistent to be called a great show, but at least it had a lot of great moments and will be seen as a very influential franchise in the years to come.
Well acroding to Isayama he did changed the ending to an extant but he's been said that first in 2017( when the post-time skip started in the manga acually) and he claimed that the changed happened back in 2014 after the anime came out and blew up. He said he was insprised to change it to a more happy ending for the genral fanbase. So the ending we got is the " happier one" to him make of that what you will. In later interviews he says he never really changed the ending but just the tone and how it was protrayed( some people don't belive him ).
Now what Eren he acuallly did allude to this turn with him in that 2017 interview as well and in recent interview with the new york times after the anime he ended he talked about him more. He said in 2017 that the ending of Aot was influnced by a manga he read called Himeanole. The manga is about serial killer who was just born with a need a murder. He had the tragic backstory and enviorment for it but none of that really mattered because in the end he was just born this way. The serial killer dosent even like this and wants to be stopped but he can't stop on his own.Isayama had said he wanted the ending to of Aot to be like this and asked the reader if a person like this is truly at fault when its just their born nature. Eren is ment to have just been born with this desire for freedom at all cost and to Eren. Eren dreamed of the outside world being a fresh empty clean state with nobody and nothing(this is what real freedom is to him)but he found the real outside world to be too like world he already knew inside the walls full of people,conflict and opression. He was disppointed and wanted to turn it to his ideal "free" world with the rumbling( he talked about this in the recent NYT interview). The" I don't know why but I wanted that" refers to him not understaning why he even wanted this "freedom" so much because he's ment to just be born with the desire and urge.
This isn't really me trying to make anybody like or dislike the ending more but to explain what I've gathered Isayama's intent here was after looking into it after being a bit confused by the manga ending in 2021. The idea Isayama had is maybe one that sounded better in his head but it just dosen't fit the series or he just didn't excute it well enough with Eren and how he was written beforehand. The ending also did have other problems as well. The anime version of the ending did make some changes thoughy by Isayama request mainly to Eren and Armin's last conversion which is even more confusing and badly paced in the manga.
Isayama's problem is that he takes a good idea from another manga and grafts it onto Attack on Titan with no thought as to how odd and off-putting it is. I never got the feeling that Eren is a mentally ill sociopath when watching the early seasons of the anime, and would argue it is impossible to see him that way even in hindsight.