@shoker
Note the gradual shift in morality.
First they fought against the titans as a mindless entity, essentially a
force of nature.
Then they learned of the
invaders from outside the walls, a completely unknown people from a completely unknown place that was easy to disassociate with.
Then they fought against their
own government inside the walls, the very people that were supposed to keep them safe, which were designated as the
oppressors.
Now, after a brief shift of focus back to the invaders, they are fighting their own friends, the people who fought alongside them just some months ago, the
traitors.
With every step, the enemy—or rather the concept thereof—is becoming closer, more familiar, and more personal. Whenever you think you've made peace with dividing the world into "us" and "them", the "us" are just divided further in a never-ending cycle of discord, and this continuous realization is eating away at their sanity.
Naturally, the last step is conquering the enemy inside oneself, which is the desire to perceive the world as "us vs. them" in the first place, in favor of just "us". All people are
us, they all deserve to live. Hence the tragedy and horror of having to kill one of
us, a fellow human being. This isn't going to go anywhere. War will either turn you into a staunch pacifist or a ruthless psychopath. The coalition group—well, at least some of them, perhaps even reluctantly—have finally arrived at the final test of their humanity.
This is actually quite poignant, and also quite bizarre that such a straightforward anti-war message is being missed for the trees.
Eren serves both as a protagonist and the antagonist of the series because of this inward focus. Both he and Zeke were early to realize how this conflict was going to end, but where Zeke's solution was one of collective redemption, Eren's is quite personal. He's the Lelouch of this series, and he orchestrated this entire endgame in a way that will, hopefully, allow the islanders to coexist with the rest of the world.