It's neither, but the anime at times is not good at feeding info to the viewer. Atalanta said she wishes for "a world where all children are loved", so her raging at children dying, even if it was necessary, is hardly out of character.
The whole point of her using Agrius Metamorphosis (something she would never use as an Archer-class) is because by then she's so angry she's beyond caring, and if it helps kill Jeanne anything's worth it.
That said, the manga does a better job at showing her corruption (iirc it's the most malevolent of Jack's spirits that egg her into murderous hatred), and I'm looking forward to her endgame with Jeanne (and Achilles) a lot, I trust Ishida will deliver.