I don’t really think Ymir went back on anything her arc built up to, I think her goal was to live a life that she could be proud of, and by saving r&b she followed her principles. It’s tragic but that’s the irony of Ymir, she always wanted to be selfish but couldn’t escape her nature of being a good person. Historia on the other hand…yeah her arc is fucking infuriating
I'm breaking my own commitment to not dip my toes into this, because impulse control what impulse control.
Here's my starting point problem with that:
Ymir does not save Reiner and Bertolt.
At best, her actions get them a pat on the head by their oppressors. They continue to live out being child soldiers for a society that considers them devils. Bertolt dies in their service almost immediately after. Reiner is a suicidal mess whose will to live is bound up in other child soldiers he's responsible for.
That's a fair tragedy, with someone trying to repay a debt only for it to amount to nothing because the cycles they're all caught up in are larger than any one personal act of altruism. Even trying to good can't undo the harm of systemic cruelty. It's a valid plot for a story like this.
Except Ymir is one of the few characters who realizes how fucked the world is. She's a better person than she ever wants to be, because being good gets you jack shit and she knows that -- but she can't help but lend people the hand she was never given. On its face, that makes her a good candidate for a hopeless sacrifice that saves no one.
The core problem is that, again, Ymir knows how fucked the world is.
You’re going to kill yourself, the ultimate act of submission. Is that how much you want to please the people who treated you like a nuisance?!
Ymir, Chapter 40
Ymir kills herself for Reiner and Bertolt, providing the people who left her with decades of living a nightmare a weapon.
Doing stupid shit to help Reiner and Bertolt out tracks. If they hadn't shown up, she'd still be in that nightmare, and she killed their friend.
But she specifically kills herself in a way that aids people who violated her, who will continue to abuse Reiner and Bertolt, and continue to launch offensives that put Historia's life at risk. Ymir has the knowledge to understand that she's not saving anyone from anything here.
There are many potential layers of story that could have been approached with this, but the bottom line for me is that Ymir's most solid convictions are all ignored when she goes with Reiner and Bertolt. There are facets you can examine to make it make sense, just as there are all kinds of things you can examine with Historia's reversal of her arc. It's always a tragedy when someone fails their principles so stunningly. It's the Bad End coming as was dreaded.
It's just that the story does not examine any of it. It's taken as a given that Ymir goes through with this, leaving us with Ymir killing herself for people who hate her in order to give Reiner and Bertolt a temporary reprieve that only condemns them to a familiar suffering.
Even then, you could make a case for characters doing stupid things if the story at least admitted that it was a ridiculously bad idea on all fronts. Our protagonist's arc is built on that. Eren makes bad choice after bad choice after bad choice and every character in his vicinity rightly goes "what." Characters can utterly fail the best of themselves and it can still be a compelling story.
With Ymir, there simply is no story. She chooses to die, and it's taken as inevitable that a character who is so anti-fate and so anti-dickheads would die in a way that benefits a "fate" she rejected and a bunch of dickheads.
Ymir kills herself, and it makes Marley happy and saves no one. She knows enough about the world to understand that.
I do not personally think that the story should get credit for tragic irony that amounts to "what if everything went to hell" without actually bothering to come up with a why for everything going to hell.
Eren's a tragic disaster; Ymir's a dropped thread.