When Victor, in Mary Shelley's novel Frankenstein, begins to collect the parts for his creature, he does so with the greatest attention to detail. Victor desires to create a beautiful creature that will deem him the best "father" in the world.
A new species would bless me as its creator and source; many happy and excellent natures would owe their being to me. No father could claim the gratitude of his child so completely as I should deserve theirs.
It is these thoughts which propelled Victor to create a being which owed both its existence and its beauty to Victor.
Upon realizing his "son" lived, Victor is horrified at the creature's physical nature.
His limbs were in proportion, and I had selected his features as beautiful. Beautiful!—Great God!
Victor had gone through great lengths to find the appropriate parts which would make his creature beautiful. His recognition that his creature is not beautiful is utterly shocking to Victor. Instead of being beautiful, the creature is so hideous that Victor cannot bear to look at him.
The importance of Victor's horror lies in the fact that Shelley desired to frighten her readers in the same way she, herself, had been frightened at the monster's creation in her nightmare. Her 1831 introduction defines her desire to frighten her readers.
O! if I could only contrive one which would frighten my reader as I myself had been frightened that night!
Shelley's use of emotive words, such as horror, ugly and hideous, are used to allow the reader to create an image of a monster so frightening that they, themselves, will have nightmares similar to her own. Given that Shelley does not paint a complete picture of the monster, only showing the yellow eyes, she allows readers to create their own image of a monster. Through Victor's recollection of the monster, this effect is compounded given he can only describe emotions attributed to fear and not actual images of fear.
The repetition of the word horror, used four times in chapter five alone, compounds both Victor and the reader's fright. Given that people tend to have different ideas of what is horrific, ugly and hideous, the use of these words is meant to evoke true fear for the reader.