Encounters with schoolyard bullies, severe illnesses, deaths of friends and family members. When I was around five or six, my mother was severely burned when taking a roast out of the oven. I remember her being in the hospital, the doctors taking skin from her legs and back to use for grafts. I remember the watery feeling in my guts when later, after she'd healed, she let me touch the brown patch of tight smooth skin on her palm. Digging into your childhood traumas might not easy, might even be disturbing for you.
Childhood is a time when everything is new, wondrous and terrifying.
A time when we feel emotions most deeply. And it's those sort of intense emotions you want to summon and use to write your horror. Pay attention to the events in the news which upset and anger you. Clip newspaper and magazine articles and keep them in a folder. Don't merely collect every article on murder you find.