Before delving into spoilers, I should say that Attack on Titan is a lot more nuanced than it appears on the surface. The story starts of very simple. But over time, as elements are introduced and the world as well as the cast of characters expands, it becomes a very intricate, thrilling and gripping.

***BEWARE OF SPOILERS FROM HERE ON!***

Attack on Titan is a series that has defined a part of me — through discussing it almost every single day for months and months... through anxiously waiting for a new chapter to drop so it'd stir the topics up a bit or finally give answers to the mystery-boxes.
Writing this review is terrifying because of how special and important this series is for me, in terms of my investment in the characters and the narrative, as well as the friends made by conversing over it.
And although Attack on Titan's latest arc, which also is the final arc, has been problematic in the way it's told the story, namely — how it has jumped from one plot-point to another, instead of slowly unrolling all of them in a one continuous flow — despite that...
I can't not say: Attack on Titan is brilliant.
And it's been so since the beginning.
At first the story purposefully draws people into having a certain mindset. But that's later revealed to be a small and narrow outlook which was shared with the characters, and that was essential to make the readers empathize with the situation the characters were in to such a degree, that it'd seal any other possible way of looking at the situation off. So when the characters were challenged in their understanding of the world, so were the readers.
At first it's just droplets of information continuing to pose questions about what is really going on in the story. But they're easy to ignore or interpret in a way that melts them into the preconception.
Then the characters start to encounter a lot more overt signs that everything isn't as it seems and find contradictions between what was their common-knowledge and what they actually see in the world.
And that results in a culmination which totally shifts the entire narrative to a completely new page and perspective. The premise of the story evolves side by side with the characters when they learn more about their surroundings and themselves. The lingering questions and clues all begin to make sense.

When you find out what Attack on Titan is truly about, at first, it may seem alienating due to the contrast it has with the story up to that point. But in truth, alien would be one of the least likely words I'd use to describe it.
What Attack on Titan shows in its last two arcs is so eerily real and close to the "real-world". The situation the characters find themselves in is something that could happen and has happened to people.
It's believable and impactful. And because every character has their own story, viewpoint and understandable mindset, it's nigh impossible to confidently call anyone right. But it also feels wrong to call them the opposite as well.
It's the ultimately morally grey story. There's no certain culprit who could be blamed for the situation in which the characters find themselves in. It's the outcome of many decisions which were taken by characters to try and have a place for themselves in the world. Who is in the wrong? Who is the villain? Who is the monster of the story? There isn't a clear answer to any such question. It's all up to the readers to decide what they see as the best path in the circumstances. It might be angering, it might make anyone feel powerlessness or sorrow... and achieving that was the intent of Isayama.
Attack on Titan is a masterpiece, it's a story that I definitely wouldn't call simple entertainment, although, it's fine to think of it as such, since it's just really entertaining too. But, if you haven't read it, please do, or not. I mean, it's just... fantastic, at least in my opinion, y'know