Isayama foreshadowed the children Maria, Rose and Sina eating Ymir all the way in 2017 with the Season 2 ED. He also titled this "From you, 2000 years ago" purposefully in order to juxtapose it with the 1st chapter's title "To you, 2,000 years ago". Genius.
So Ymir never made a deal or anything. We still have no clue whatever that Spinal Creature thing was that dwell within the tree. Aliens I guess.
Essentially Eren's philosophy comes down to this, Ymir was a slave herself her entire life to the Royal Family. But it didn't even end there, the entire bloodline is contaminated with the same ideology. Everyone who is an Eldian was born into the life of slavery - remember that Eren said that he simply wants to be freed. But they can never be freed - they're chained by the past from the very start. Ymir doesn't just hate the People of Marley but she also detests King Fritz and what he did to her, the people who betrayed her from the start.
So anyone who is an Eldian will never be freed. That's why Eren is acting the way he is, by freeing Ymir first. It also reminds me of Fate/Zero.
Spoilers, but hey it's been out for years
Kiritsugu wants to save the world and become a 'hero of justice' but finds the world to be cruel and unjust. As a result he murders countless for the greater good, then hearing about the True Miracle that the Holy Grail can offer - becomes a mercenary for the Holy Grail War. Once he comes into contact with the Grail he asks that everyone be saved. Kiritsugu finds to his horror that the Grail is corrupted (due to previous events not in this series) and that it can only accomplish his wish through the means that Kiritsugu understood. In other words the Grail would kill millions of evil people around the world to save those who were good. It then started making Kiritsugu choose: Would you save a boat of 500 or a boat of 400 people? 300 vs 200? 200 vs 100? Would you save your wife or your work partner? etc... Leading up to the Grail tempting Kiritsugu with the happiness of living with his Wife and Child. Kiritsugu said it wasn't even an option, and proceeded to break his Daughters' neck and kill his wife - choosing instead to destroy the Holy Grail preventing it from performing his wish.
And you might be asking how it's similar. Well assuming that these are in fact Eren's true feelings and his goal, then he's practically the opposite of Kiritsugu morally. I think it's interesting he would choose a single individual over the many.
So everything that happened between Zeke catching Eren's head and Eren shooting the new spine out happened in an extremely short time frame. Kind of silly, but at least we're going to be seeing something new and freaky next month.
Also, Isayama did some good research. Using the extinct hallucigenia as the tree creature was a neat choice.
