if RtS was reversed, and Erwin lived instead of Levi, the expression on Levi’s face that’d stick with him for the rest of the story - like Levi w/ Erwin’s smile - would be the look Levi had the moment they were alone right before their argument.
Erwin feels so much self-hatred and shame just… at his baseline that Levi’s death would just annihilate him emotionally. in that way, he’s dead too.
at best he’d think Levi pities him for letting him die (regardless of how much that’s actually based in reality), and at worst Levi regrets having anything to do with Erwin or the Survey Corps, but most of all he resents Erwin for “tricking” him into it. (this is infinitely more painful than canon RtS eruri somehow).
all he could fixate on, towards Levi, is the pain he ended up putting him through just to get the best hope for humanity killed (again regardless of how relevant that is…). and… Levi, killed. Levi who put all his faith in him. *10 page essay of everything Erwin admires about Levi*.
so this moment stands out in his memory the most. what if he had stayed? or what if he gave them both a different reason to fight for in that moment? - when Levi asked him whether seeing the basement is the most important thing to him.
it doesn’t really matter, but that’s the single last time he saw Levi upset by his actions, probably the only time it was so clear, and raw, abject. it was just the night before. of course Erwin finds that to blame himself for.