Attack on Titan Season 3, Part 2 may have well been the best season of the series yet, delivering some of the best moments this anime has ever produced. Among them was Erwin's final speech and charge towards the Beast Titan, which was probably the anime's most tragic death scene yet, while the famous "serumbowl" scene may go down in anime history as one of the greatest scenes of all time. The latter, in particular, prompted many fans to wonder why Levi chose the rookie Armin Arlert over the veteran commander Erwin Smith, and no -- it is not because of plot armor.
The scene in question was foreshadowed even before the battle of Shiganshina began, and the decision Levi ultimately decided to make was both layered and consistent with the plot developments of the show. Attack on Titan is brimming with complex characters making complicated decisions, and serumbowl is a perfect demonstration of this. Levi's decision to let Erwin die was multi-faceted and contains more reasons than one, plot armor being the last of those reasons.
Armin's Dreams and Potential
The first foreshadowing of Erwin's death came when Levi and Erwin had a discussion prior to the beginning of their mission to reclaim Shiganshina. Levi asked what Erwin planned to do after he found out the secrets in the Jaeger family's basement and, by extension, the secret of the outside world. Erwin replied that he didn't know. In the battle itself, moments before Erwin's demise, he confided in Levi that he only led countless of his scout members to their death only because he wanted to know the truth of the world, and to confirm if what his father told him was true.
It was Erwin who entrusted Levi with the syringe containing the Titan serum, and when the moment came to choose who to use it on, Levi realized Erwin no longer had any more ambitions to lead the scout regiment after he found out the truth from the basement.
Armin, on the other hand, still holds ambitions beyond the basement's answers. Ever since he was a child he dreamed of being free and traveling to the outside world to see its oceans and deserts in all their glory. He is still a rookie, but a young, bright soldier full of potential cannot be replaced. And no, it wasn't Eren's emotion and heart-wrenching speech to Levi that convinced him to safe Armin instead, the seeds of that were planted even earlier.
Right after Erwin's group secured the reigns of the government of Paradis in a successful coup that crowned Historia the new queen, Armin, Mikasa and Eren are shown having a moment together in the night. A starry-eyed Armin enthusiastically reminds an upset Eren about their dreams to see the oceans, and then the camera cuts to reveal a somber Levi secretly listening in to their conversation, foreshadowing the decision he eventually has to make.
But it also wasn't because Levi thought Armin would be a better choice than Erwin in their fight to protect humanity that made him pick Armin over the experienced commander. There is, after all, no certainty that Erwin would stop fighting for humanity after knowing the truth in the basement. Likewise, there is no guarantee that Armin would live up to his own potential. At the core of Levi's choice, deep down, are his emotions and love for his friend Erwin.
Erwin: The Devil That Will Save Humanity
In the aftermath of Erwin's tragic suicide charge, and right before Levi was hesitantly about to use the syringe on a dying Armin, Floch shows up carrying the comatose body of Erwin. The surviving Scout member proceeds to deliver a speech about how an expandable soldier like him managed to survive only because it was his destiny to carry Erwin's body to the Titan serum. He tells Levi that Erwin is a "devil" that led many to their deaths, but that a devil is what humanity needs to attain their freedom from this "hell." Floch's argument for saving Erwin not only resonates with Levi but also ironically has the opposite effect on him than what Floch intended.
When Levi attempts to use the syringe on Erwin, the Commander unconsciously slaps it away and mumbles in his dreams. Levi sees Erwin's peaceful face and realizes that, as a friend, he could not forcefully resurrect Erwin and make him relive this "hell on earth" a second time. After all, what kind of a friend would wish for another friend to suffer and play the part of the "devil?"
With his mind made up, Levi finally makes the difficult decision of injecting, quite literally, a second life into Armin, while letting his old friend Erwin rest peacefully for the first time ever since he became Commander.