The characters of Titan naturally have to hold on to certain dreams in order to keep going, though Kenny likened that to being a slave to something or someone. For Levi, that has become serving Erwin and seeing to it his dreams come true, if at all possible. That’s why, when he appears with the syringe of Titan serum, he holds out as long as he can to give it to Armin, who is on the verge of death.
Eren is slave to Armin’s and his dream of seeing the ocean one day, so their opposing dreams create a conflict. As for Zeke, he leaves Bertholdt for dead and lives to fight another day, promising Eren, fellow victim of their father, that he’ll come back to rescue him some day.
On the other front, Hange sends Mikasa to see if Levi is alive and still has the serum; if injected into one of them, they can eat Reiner and gain his powers. Otherwise Hange will kill Reiner right there and then, but not before taking possession of a letter from Ymir meant for Christa.
When Mikasa meets up with Eren and Levi, and sees what has become of Armin, Floch arrives just before Levi administers the serum to Armin, with a badly wounded but still breathing Erwin. Right there and then, Levi changes his mind; Erwin will get the serum. Armin will die.
It’s an impossible choice, as both Erwin and Armin have proven to be an integral part of the continued survival of humans within the walls. For Levi, it’s a simple matter of giving it to the more experienced leader, but he can’t pretend he’s not influenced by his greater loyalty to his commander, as well as his buy-in to Erwin’s dreams.
When he knocks Eren aside to execute his decision, Mikasa pins him down, and because he’s in such bad shape from the Beast Titan fight, Mikasa can tell she can overpower him. But Floch is on Levi’s side, and tells them how he believes was the only one of his unit to survive so he could ensure the “devil” Erwin couldn’t go out so easily, that he’d live on and continue to be the devil they needed to defeat the Titans. Then Hange, Jean, and Sasha drop in, further complicating matters.
It’s an apt microcosm of the inner-wall humans all along: different factions or individuals wasting precious time and resources fighting over which way to proceed, with Levi, Hange, and Floch on one side and Eren and Mikasa on the other. Both are factoring emotions into their decisions, and both are right that if either person dies it will be a huge blow to the cause.
Ultimately, when everyone has cleared out, Levi is about to insert the syringe in Erwin’s arm, and Erwin suddenly moves it out of the way, as if to prevent the injection. Turns out he’s delirious and raising his hand as if he was still in Grisha’s classroom. Levi takes it as a sign it’s time for Erwin to finally get the rest he’s deserved.
The next time Eren sees Armin, he’s a Titan, gobbling up Bertholdt and gaining his Colossal Titan powers. Levi quietly explains his choice, and begs Floch and everyone else to forgive Erwin, who never wanted to be a Devil. Everyone made him the devil, and it didn’t seem right to pull him back into Hell when he was finally freed from it.
Instead, Armin is being given a second chance to see the sea with Eren and Mikasa. By being given the abilities of the biggest, strongest Titan, he’s taking Erwin’s place as the Devil the humans within the walls need to lead them to victory.
Even with this massive shift in the balance of power (the Titans’ greatest weapon we know of is now in human hands), even dragging Armin back into Hell might not be enough. There’s still so much they don’t know. But with Zeke and Reiner in retreat, at least the path to The Basement now seems clear.