It isn't uncommon for humans to be seriously injured during an Eternal invasion, whether due to self-defense or accidents. When a human is on the verge of death, it depended on the individual Eternal if they were to be saved before the world was rescinded. If the humans died before then, then they'd still be stuck in the cycle of reincarnation.

    Here, Maybell was confronted with the choice of saving a human that she had shot when he had leapt out at her so suddenly, trying to attack her. It tended to take more energy for transformations that required less time, both in body and in mind. That could prove costly (personally) in the beginning of an invasion, but she couldn't stand the thought of being so callous.

    When Maybell completed the transformation (thus saving the ex-human's life), the new Eternal found her Arivida a bit exhausted, being relatively new to the life of an Eternal herself. She felt conflicted, but otherwise assured that her Arivida loved her. She didn't know what the deal was, as far as the invasion was concerned; but, she could stand to support someone who saved her life and clearly loved her.