What is scary about Frankenstein?
The fact that Victor Frankenstein has created a seven-foot tall creature out of dead body parts, a creature that is quite willing -- eventually -- to kill and frame others if it suits his purposes, is meant to be scary. Further, he has superhuman speed and superhuman strength, and there is really no way to escape him if he sets his mind to catching you. Imagine such a creature setting foot inside your home or your classroom; it would be pretty horrifying!
Also, Victor's loss of humanity as he works on his experiment is meant to be pretty scary as well. He goes into morgues and tombs, looking for fresh body parts. He says, "often did my human nature turn with loathing from my occupation [...]." The work he was doing was so awful and disgusting that he actually had to shut off his natural human tendencies in order to proceed. Anyone who can so turn off his humanity, who is willing to meddle in affairs of life and death with "profane fingers," is pretty frightening as well.