With the Attack on Titan Rumbling arc starting in the anime, I think it's important to keep in mind:
Whether intentional or by a very unfortunate and badly-thought analogy, SnK perpetuates antisemitic tropes. It doesn't matter that the eldians are "displayed in a sympathetic light". The Holocaust imagery, like the armbands, does create a clear link with Jewish people. By making Eldians capable of transforming into man-eating monsters, the manga/anime references the blood libel, an antisemitic conspiracy that has been historically used to dehumanize them.
The manga/anime is very popular in Alt-Right groups. There is a clear reason for this. The philosophical question that SnK poses about the division between "the nation and the enemies" is not resolved or elucidated and serves itself as a fascist thinkpiece. (in other words, it isn't directly fascist, but it starts deconstructing fascist ideas and then just stops, leaving the story in limbo.)
Yes, it's fiction. Yes, it can be consumed critically. Different political ideologies are explored in the manga/anime and, interpreted in a certain way, the story although pessimistic, tries to give an anti-war message. However, it cannot be consumed apolitically.
Isayama does his damn best in the last arcs to make genocide seem like the only option. Many other (and better) people on the fandom have already mentioned that diplomacy is never truly considered and that the story corners Eren into a Rumbling.
The problem is that SnK, by trying to be neutral, ends up endorsing very horrifying worldviews. There is no justification for genocide. Any excuses should have been rebutted as explicitly as they were expressed in the manga/anime. But they are not. Of course, there are characters in very explicit opposition. But their vision is offuscated a narrative that forces a lack of peaceful solutions.
Fiction is fiction, I agree. However, fiction also transmits messages, and it's integrated into the large fabric of social discourses. You can watch AoT. But if you keep insisting that there are no harmful elements in the story, if you call yourself a "Yeagerist", if you advocate for the 100% Rumbling (or any other Rumbling), it's time to reflect on why you condone it.