I think it was because I was still kind of little, and I saw this adorable little boy who wasn't much younger than me, who looked so normal and unassuming, doing completely evil things all the while not looking evil (but isn't that a boy for you, ha!). The hanging and balcony and lightning rod scenes really did scare me. And for a long time after watching that movie, I always secretly wondered if the boy sitting in front of me in class, or the one who looked at me as our parents' cars drove past one another, was the antichrist.
I think I would have actually asked my mother if she thought this one, or that one was, but I knew if I did, she'd never let me watch another horror film again. I had to learn to just keep my thoughts to myself and my mouth shut!
Image source: Wikipedia