A lot of authors start with the premise "What's the worst thing that can happen?" I think so many children's stories start out with dead/evil mother's because even today, for a lot of people mother = primary care giver. A child may adore dad, they might even like their father more than their mother, but it's mom that inspires panic when they are separated. It's hardwired into all mammals that losing mom or not having a mother's love is the worst thing ever.