I think it's more of a horror. The very idea of a re - animated corpse made from different body parts is a classic horror trope. The way the monster is often depicted as a menacing, unstoppable force is typical of horror. Although there is some science in the story about creating life, the overall tone and focus on fear make it horror.
I'd say Frankenstein leans more towards horror. The idea of a monster brought to life and the terror it causes is a classic horror element. But it does have some science fiction aspects like the concept of reanimating dead matter.
It could be either. It depends on the specific elements and themes presented in the work. Some extant works may have elements of both horror and science fiction.
Frankenstein is often considered both science fiction and horror. It has elements of scientific experimentation and creation, which are typical of science fiction, but the terrifying and monstrous aspects make it horror as well.
Well, think about the main focus of the story. If it's more about future worlds, scientific advancements, or alien encounters, then it's likely sci-fi. But if the emphasis is on creating fear, monsters, or the unknown that terrifies, it's probably horror.