There are any number of explanations that could apply. You’re presenting a false dichotomy between “mental illness” and “some people are just evil” because you’re looking to silence opinions you do not agree with, and you don’t have a right to do that. What if there’s a different reason? What if she was blackmailed or brainwashed? What if it was part of her religion? What if someone paid her off to do it? Your mind jumped immediately to mental illness, a mindset that stigmatizes the mentally ill. You cannot call yourself good or just when you do that.
The truth is that some people really are just evil and they do stuff like this because they’re sadistic, or entitled, or because they think they can do what they want with impunity. You being unable to accept that fact doesn’t make it not true, and that answer not sounding pleasant to you doesn’t make it false either. No one’s obligated to give a baby murderer room for change; they’re allowed to put the well-being of children and the community above hers.
It sounds like you’re just mad because other people are rightfully mad at her and want to see her punished, and you can’t see what’s wrong with that. You legitimately don’t understand that it’s not right to morally condemn others for wanting to excise someone like her from the community – an act hardwired in most social animals to protect their safety and the safety of the group, including humans. Maybe the one who’s mentally ill here isn’t her, it’s you.