There was a big argument over who should receive the titan serum syringe: Armin or Erwin. But why does Armin need to turn into a titan in the first place?
Armin had to turn into a titan so he could eat Bertolt. If a titan eats someone who has one of the Nine Titan powers, they will revert back to a healthy human and steal that power as well. Armin was on the brink of death, so this process was the only option to save his life.
He was gravely injured during a sacrificial plan to distract the Colossal Titan.
Chapter Behind Why Armin Had to Turn Into A Titan
In chapter #82, Armin splinters off from the Scouts in order to perform a plan he has to take down Bertholt, the Colossal Titan. Bertolt plans to destroy the wall of the Shiganshina district. That would putt a massive hole in the Scouts’ plan to retake the area and safely enter Eren’s basement.
Reiner and Bertolt’s goal, aside from destroying the scouts, is to capture Eren yet again and return him to Marley. Bertolt reflects on this goal as he stares at the beaten-up Attack Titan who barely clings to consciousness on top of the wall.
“I came up with this plan myself,” Armin says as he sits on Eren’s shoulder. “But whether it’ll work or not is going to depend on how much I can take.”
Eren starts to realize that Armin may be making a sacrifice of some sort. Armin confirms this suspicion. Yet, he assures Eren that he’ll “stop before things get really bad” and allow for Eren to finish things off.
Armin tries to calm Eren down. He assures his friend that he still intends to keep their promise of going to the ocean together.
Armin’s Plan
The plan is then set in motion: Armin distracts Bertolt by intentionally jumping into his extremely hot steam. He connects his ODM gear hooks into Bertolt’s teeth, allowing him to not be blown away. Armin also is able to confirm his hypothesis that Bertolt can’t move while emitting the fumes.
Bertolt is completely confused by Armin’s plan. He questions the logic behind the diversion. Eren falls off the wall and is knocked out on the ground, and Mikasa and Jean have their hands full with Reiner.
Armin’s plan then becomes clear. Eren’s “knocked out” titan is actually a hardened dummy version of itself, and ODM gear-Eren sneaks up behind the distracted Bertolt and takes him out. The plan was a success, but Armin was absolutely burnt to a crisp.
Transformation in Next Chapter
The following chapter, an argument ensues over who will receive the one and only vial of titan serum: Armin or Erwin. Before the retaking of Shiganshina, Erwin explains to the scouts that should a soldier be on the brink of death, they can turn them into a pure titan with the serum. If that pure titan eats a user of the Nine Titans (such as Bertolt) they will revert back to a healthy human and steal that titan power as well.
In the end, Levi chooses to save Armin over Erwin. This is decided because of a myriad of reasons. But, the basis that Armin has to turn into a titan is so that he can eat Bertolt and revert back to a healthy human.