How to Write Horror/Thriller/Suspense [idk where to post so please move if needed]

[Snatch Steal]

This evening, I read A Good Man Is Hard to Find by Flannery O'Connor, a short story she wrote in 1953. At first, I thought the text was flowery and outdated, and just another piece of oldies crap that used passive voice and unnecessary adjectives as frequently as possible, but a bit over halfway through the tale I found my head throbbing from the suspense.

To paraphrase, a grandmother prepares unwillingly to drive to Florida with her son, his wife, and her son's children. It's pretty generic at first, a good-old family drive through Georgia (where she says some awfully racist things), when the grandmother remembers a plantation that she visited as a child, and when they take off on a dirt road, the family ends up in a car crash. Finally, a criminal, "The Misfit," who the grandmother forewarned them about, finds them and one by one takes them into the woods and kills them; Bailey (the grandmother's son), his wife, their children, and even the grandmother herself.

To say the least, I got sucked up, and finished reading at least ten minutes faster than I usually would (even with annotations). Suddenly, I wanted to know whether these people lived or died, especially considering how innocently the outlaw approached the family, and as signs arose of their demise, I felt helpless, as if I desperately wanted them to escape, but they had no hope. The inability to help, and their helplessness, made me squirm.

What other aspects of horror have you observed, such as recurring themes or devices? If you enjoy horror, why? What makes horror truly horrific?
Also, considering this wasn't a particularly "horrific" story, could that tie in to why it was shaking?