As a child movies with monsters like Gremlins and Critters terrified me.
As an adult, any movie where a baby or young child is killed is too much for me to handle. One of my kids is the same age as Gage from Pet Semetary (and just as energetic), and that character's death scene from that movie is almost every parent's worst nightmare. When Stephen King wrote that book, there was a real life instance that inspired the whole story; as he and his family were outside, his then two year old son ran towards the road. Stephen caught his son just in time before a huge truck sped by. He lay in bed that night thinking to himself, "what if I was too late", and Pet Semetary came to be.
I also hate seeing people get decapitated/their heads mutilated in movies. It may not be the absolute worst way to die, but I think it's th most gruesome-looking way.