

I think the primary theme of Attack on Titan is "idealism vs harsh reality of this world". To understand it, let's get through it from the beginning:

The story, just like for each of us, begins at the childhood.
The world is quite fun and simple. We don't have any bigger issues, because adults are keeping us away from serious topics. It might feel almost like all problems of this world were kept behind a thick wall.

As children, we are very optimistic. Guided by positive things like seek for knowledge, exploration, freedom, independence. We want world and society to change in a positive direction. We want to confront the evil without taking into account any negative consequences.

We're getting irritated by people who don't want things to change. It seems like they're so lazy, that they want just to have fun, drink alcohol and play cards.
Depending on whom (and which "side") you are, you might get angry at:

The things change when we experience the brutality of evil first-hand. For Eren it was the most terrible thing that could've happened:
For you, it doesn't have to that awful, but for sure you've experienced negative consequences of evil somewhere in your life. Once the wall of optimism is broken, there is no going back.

For the first time in our lives, we feel new powerful emotions - uncontrolled anger, wrath, malice. We now not only want to overcome the evil, but destroy it, "kill every single one of them"!
The general rule of the thumb is: the more awful things the evil makes to you, the more radical about eradicating it you will get.

After we had been radicalized, we want to fight the evil. We quickly realize that our determination and anger gives us fuel for being successful. Using it, we can even overcome our lack of skill and talent.
You finally start to think you can defeat the evil.

However, we soon realize that the fight with the evil is hard and painful. We are completely powerless, shocked by the price we have to pay.

And then we finally get the power we needed to defeat the evil.
I think that the 10 Titans represent 10 skills that can help us fight:
I still have a problem figuring out which skills War Hammer Titan and Jaw Titan represent.

Having the power makes other people fear us.
(while others wants to exploit us, because they see how beneficial our power can be)

The society is not grateful for our sacrifice.
We feel lonely and betrayed. Outcast without his place in the world.

And then we find and join forces with people who have similar views to us, who are just as radical as us, who we can trust, who have the same goals as us, we feel like they are "just like us".
They have all features we always needed, there are people who are more skilled and talented than we ever will, people who have knowledge and expertise we always lacked, a charismatic leader who is smart and capable. We start to adopt their symbols, we finally have a feeling of belonging.
By their side, you feel like you can finally achieve your goals. You feel just like you had wings on your back.
The equivalent of it, in the real life, might be joining some far-right nationalists group or something like Antifa.

The notion at this time is quite simple - good "us" vs evil monsters. We don't see them as humans (even dough, they for sure look like them), just mindless animals who have to be exterminated.
The rest of the society is "blind", "stupid", "passive", "controlled by fear", they are just an obstacle in our fight.
You feel like you are the force of justice, a hero, every sacrifice of one of your men only increases your sense of vocation.

But after some time you find something terrible. Your worldview was too simple.
Beside mindless monsters, there are also some people who aren't just stupid or straightforward evil, but rather have different goals and interests than you have (which you can't even understand yet). They might even be one of "your" people.
I think it's very similar to the real world, where both radical left and right are manipulated by various interest groups, who are using them like tools for achieving their goals.
With that knowledge you feel even more angry, betrayed and confused.

You remember how I talked about 10 Titans representing 10 skills useful for fighting?
There is one power that exceeds all other powers you can get by a high margin. That makes you unstoppable. Power of kings. Power of gods. Power to rule over the people. The power which is owned only by kings, dictators, presidents, governments.
Just think about what the coordinate allows you:
This power is holy, it's mythical, it's divine, it's mysterious and... it comes with a price.

So you have now started the revolution.
Together with an influential general, you got rid of the bad, corrupt, passive, lying government. You've broken the status quo, destroyed the current order of power. But is it necessarily good or bad? Well, it's hard to tell...
The old king was keeping the relative peace and prosperity for a long time. But would it last any longer when there are outside forces who are getting stronger day-by-day and want to destroy the country? Maybe the king would be able to eventually destroy all the enemies by himself? Maybe you are the one, who is just making it harder (or impossible) for him? Or maybe the enemy agents would eventually defeat him and took over the state?
It's very hard to judge, but most likely some action was really needed.

All kind of rulers are usually just guardians of the status quo. They don't want any radical changes, because they threaten their position of power and prosperity of their nations. They do this by using various unethical techniques, like hiding the truth from the people.
For people from outside, it might look even like they were possessed by a some kind of curse, which makes them passive pacifists. The true change is then possible only when the power lands in the hands of someone who isn't possessed by that curse, someone who isn't even from the royal family.

Then we slowly start to realize the most terrible part of being a person in power - the concept of "greater good". Have you ever heard the phrase "the end justifies the means"? It's a very hard moral dilemma, but the problem is that for a ruler, the only possible answer to this question is a big fat YES.
A governor who wouldn't use right means would never achieve any goals and his country would fall into chaos.
Sometimes a good commander have to do things so terrible like lying to his own people and sending them to certain death in order to achieve the greater good (like defeating the Beast Titan, taking back the walls and getting to the basement).
There is even worse part of it - sometimes you won't even achieve the goal you wanted to achieve, but the price have to be paid anyway. (in this case, the Beast Titan managed to flee)

And then one day comes a harsh realization. If the only thing that really matters is the outcome, and none of the moral standards, laws and rules apply to the people in power, then in fact there isn't a single thing that is so terrible, that it wouldn't be worth doing for the sake of the "greater good".
I think this is the biggest misconception people have about the plot, that they think, there was "a better way to achieve the same outcome (global peace)" or "there for sure was possibility for a better outcome". NO!
It is explicitly mentioned that Eren had a power of seeing all the possible outcomes. All the possible versions of the history.
He has seen all of this and decided this version of the history is the "best outcome" (by whatever subjective criteria he used to judge) and there isn't any other way to achieve it.
Just imagine it, you become a king of your country, and then you realize that the only way to preserve peace and prosperity is to:
This is just awful!

The truth is devastating to the soul. You feel like it wasn't what you were expecting.
At the beginning you started, because you hated evil and wanted to be good, wanted things to change for good, wanted to explore the world and have fun. And now you realize you cannot just do whatever you want, because you don't live in a vacuum. There are other nations, who have different interests, the nature of existence in inherently a constant conflict.
The "monsters" were all along just other innocent people, and you cannot save them all.
You wanted freedom above all, but now you've become slave of your own knowledge and duty, and there is only one path you must follow.
You wanted to know the truth, but it is something completely different than you expected.

The power makes you bad and evil.
You become so obsessed with your end goal, that now and here is losing its meaning. You might even become ruthless and unfriendly to your own family.
The insane stress caused by the path you've chosen is slowly making you lose yourself.

Let's save the world! Probably the most cliché story possible...
But yet this is the new breath of optimism in a world of total war and chaos. The common ideals of all the people. The fight for survival of the humanity, which is a goal above all differences like nation, race, gender, age and background.
Truly beautiful.

The happy ending is there after all. Just not for you...
Was Eren a bad person, because he has murdered half of the humanity?
Was Eren a good person, because he eventually brought peace to the world by sacrificing himself?
It's really hard to tell.
AoT doesn't give the answer, because there isn't one.
Moral judgements in some cases are really complicated and non-linear.
And that's pretty much it.
AoT've really got the best ending we could've asked for. Most people don't like it, because just like the reality it's harsh, it's non 0-1, it's unexpected, it's hard to understand and for many disappointing.
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