It’s interesting to me just how rare the underlying mental disorder has to be. Millions of people have mental health issues too but aren’t committing unspeakably vile acts. The incidence rate has to be 1 in several hundred million births.
My other thought is that mental health played a role but isn’t the underlying cause, since mental health problems generally don’t drive people to do this. That said, with the complexities of genetics and epigenetics, it’s perfectly possible it could happen.