Yup. My next-door problem child is from a blended family.
Still, this kind of thing does happen occasionally in intact families.
A few years ago in NY there was a little girl who killed her newborn twin brothers in a jealous rage. She took them out of their cribs and smashed them against the floor.
IIRC this happened in an intact Italian-American family. (Though I wonder if the parents are still married today --could any marriage survive such a tragedy?)