Superficially, Spider-Man and Brundlefly are very similar. Spider-Man results from a fusion of spider and human through an accident of genetic engineering. Brundlefly results from a fusion of fly and human through an accident of genetic engineering. Spider-Man and Brundlefly both gain similar powers: super-strength; the ability to climb on walls. They both have powers specific to the invertebrates with which they have fused: Spider-Man secretes webbing; Brundlefly secretes acid.
Where Spider-Man and Brundlefly differ is in their degree of deformity and destructiveness. Spider-Man is not deformed; he is a good-looking young man who not only remains good-looking after his accident, but develops a better body. Brundlefly becomes hideously deformed through a degenerative process that ultimately leaves him looking more like a fly than a human. Spider-Man's one slightly icky characteristic, the webbing that comes out of his wrist, is neat and contained. Brundlefly's acid secretion takes the form of vomiting, and is anything but neat and contained. Spider-Man could be destructive, but uses his powers to fight crime and save lives, so he is not destructive in the sense of destroying what shouldn't be destroyed. Brundlefly uses his powers to maim people and attempt to fuse himself genetically with his girlfriend and unborn child, so he is destructive.
From this analysis, it should be apparent how central deformity and destructiveness are to the horror movie. With a slight adjustment, the movie Spider-Man could be turned into a horror movie: if Peter Parker became gross-looking and used his powers for evil and tried to fuse genetically with Mary Jane Watson. And The Fly could have been a superhero movie if Seth Brundle had remained good-looking and secreted his acid more discreetly and used his powers to fight crime.
What is most amazing is that both movies are enjoyable. Movies are fantasies of choice, and some days I might choose the fantasy of the superhuman good guy, other days the fantasy of the subhuman monster. As long as both movies are well made according to the rules of their respective genres, both Spider-Man and Brundlefly can end up winning this contest.
Deformed and Destructive Beings: The Purpose of Horror Films