What I don't understand is what triggers Ellie her sudden drastic decission. I don't really remember what happened exactly.At least is very bad "written". She was totally insane, almost berserker, fighting for her life and integrity and her mind totally commited to kill at last that bitch Abby. We experienced her killing in cold blood a lot of people, but suddenly in that high-rage and survival moment has a flashback of Joel (that would enrage her even more) and decided to stop?
Sorry, or I'm missing something (I must rewatch it) but I don't buy your interpretation.
Suffice to say I do like the game and don't share some critics people are doing, but c'mon, that end is nonsense. The writer could came with a better solution for letting both alive without using that shit. Remember, two minutes before that she was coldly putting her gun in Lev's head. She could had flashbacks or the many murderings and her miserable path to reach this moment, that would be a trigger! She even could saw as a redeeming view the Abby-Lev relations as similar to Joel-her, and even how abby let her go several times (addet to all the previous reasons).
But no, what makes her change drastically her behaviour was a smiling Joel playing the guitar.
Sorry, I insist. The most consistent ending for the moral of the story and the development of the characters would be both dying.