Looking at Levi's expression when Floch brought Erwin back, it's clear that he was surprised Erwin was still alive. There is hope in those eyes, Levi thought he had a chance to save Erwin. His first thought is then about how he could ensure that Erwin would be okay.
The look of desperation on Levi’s face, he really wants to save Erwin.
He clutches the serum to his chest knowing that it is Erwin's only chance at living, holds it to his heart as if to stem the pain he must feel at the thought of losing Erwin just after he had the chance of bringing him back
How he looks at the injection. It’s the only thing that can bring Erwin back, and he looks desperate to save Erwin.
He looks so pained whilst preparing the titan serum and listening to Hange, how they talk about how everyone leaves you, but he has the chance to not let the person he has cared about the most leave him, and he hurries to ensure Erwin’s life. He understands the pain, having lost so much people, and he doesn’t want a repeat of that with Erwin, becuase he knows it will break him.
When Eren grabs his ankle, and starts talking about Armin, and his dreams, he rembers Erwin’s dream. It is here we can see conflict within Levi, he wants to bring back Erwin for himself and humanity, but he knows how Erwin was obsessed with that dream, how he didn’t know what he would do after, he just wants the best for Erwin.
You can see the pain in his eyes, he wants to bring Erwin back to life, but he realises that Erwin deserves to be at peace as well. That he has fought so much and given up so much, that he now deserves to be at peace.
Levi has a flashback to when he and Erwin was talking about his dream of seeing the basement. How Erwin’s goal seemed to end at the basement and knowing the truth about titans.
It can be interpreted that maybe Erwin's dreams have changed. That maybe the most important thing to him had changed. From discovering what was in the basement, to a person. Levi was the most important thing to Erwin, and he wanted to ensure in his dying moments that he was safe.
He also remembers Kenny’s words, how eveyone had to be ‘drunk on something’ and Levi realises that Erwin was so focused on his dream and find out if it was true, what his father thought.
At first, Levi wanted desperately to bring his Erwin back, but then he realized the pain, guilt and depression that haunted him.
Levi realises how human Erwin is, that he deserves peace and rest after sacrificing so much. He didn’t want him to become the devil, or a titan, and chose to let him die as a human.
He acted selfishly, choosing this for himself and Erwin, to let the man he cares about so much finally rest. It demonstrates the amount of love Levi has for Erwin, that he’s willing to let him go for his happiness.
‘ Erwin was a slave to his dream. As long as he is alive, he will never get over his dream and death is the only way to free him. Levi giving up the chance to save Erwin for himself and realised Erwin from being a slave to his dream. Levi understood Erwin's feelings. ‘
Levi not giving Erwin that serum was him showing his love for him. He let him pass on as a human rather than drag him back as a monster.
Erwin pushed Levi’s hand before he could inject him with the serum. Levi was not expecting Erwin to open his eyes or even move, he thought Erwin was too far gone to come back to him. Yet Erwin did, he came back to have his last moment with Levi.
Erwin fought to come back and speak to Levi one last time. He came back to talk to the person he loves. To tell him that his childhood dream would be fulfilled by Levi, but that he wouldn't see it. That he wanted to pass on. But he ensured that first, Levi was safe.
And for Levi, he let Erwin pass on peacefully. He let Erwin finally rest, because Levi knew that Erwin was in some ways satisfied with the information he knew: Levi was safe, and humanity had a chance because their strongest soldier was still willing to fight.
Erwin trusted that Levi would carry on for him. That he would continue to fight for their dreams. Erwin knew that Levi's dedication to him was something more than just Levi owing him for saving him from the underground. That it was both a shared love (of each other) and a dream.
‘ Kenny didn't use the injection for himself but gave it to Levi. For Levi this was a totally selfless act he never thought a man like Kenny would ever do. Influenced by this experience, Levi chose to not restore Erwin to life but treat him as a human and let him go. ‘
‘ Levi lost Erwin, who is someone irreplaceable to him ‘
‘ Levi's like a balloon dangling in the air. To Levi, staying with Erwin until Erwin's last minutes gave him a feeling that he has accomplished everything he needs to. Of course Levi still wants to kill the Beast Titan. ‘
Levi went back to collect Erwin's body, to let him have a final rest in a place he deemed suitable. They held a special memorial service for Erwin.
