First, I would suggest that this question be moved to the Discussion section of eNotes. I am sure that you will receive many excellent, and differing, views on what makes Frankenstein horrific.
As for my own personal point-of-view, I believe that the novel Frankenstein is a horrific novel based upon the fact that Victor Frankenstein succeeds at reanimating life.
The fact that the possibility of reanimation exists is frightening. The fact that Victor, albeit in a fictional novel, is successful in reanimation is horrific based upon the fact that it could be used to support a person's (in "real life") own obsession to do the same.
Outside of that, the horror is compounded by the fact that a "father" could alienate his own "son." Today, we see many parents abandoning their own children (normally based upon the fact that they did not chose to bring them into the world). In regards to the novel, Victor chose to bring his "son" into the world. He obsessed with it, alienated his own family, and allowed his own health to deteriorate.