>>7 I believe it's an exaggeration of the common dominance/submission trope which is appealing to most humans on a primal level. Also, physical reactions to pain are often similar to pleasure (the adrenaline rush etc.), and while I'm not suggesting those who enjoy torture fic do so for only (or even primarily) sexual reasons, the correlation could be a contributing factor. Besides that, for those who enjoy language, there is a great capacity in torture/gore fics for lots of visceral description and intense metaphor. /lecture