Kahn now lay besides Kassandra whose condition seemed to be worsening with each passing hour. And after a reminded from Armin about the procedure being painful… Kahn braced himself for the agony he was about to go through soon.
There were two main reasons why Kahn made this decision to save Kassandra even at the cost of sacrificing a portion of his soul.
One of them being regardless of her reasons… the fact remained that Kassandra had indeed saved Kahn's life.
Because even if he had somehow managed to live after getting hit with that soul attack from Caladrius… he'd either faint or be in a helpless state where the legendary monster could simply kill him with any of its wind elemental skill.
Even Armin would not have been able to protect him in that state because Blackwall and Oliver had already fainted and he himself had no offensive skills or hiding skills to protect Kahn.