Using ghouls in place of other monsters – particularly to show the corruption of a specific race – is a solid idea. In this particular instance, it wouldn’t be out of place to use it as an illustration between fey and the shadowfell or demons or whatever. The unnatural nature of the ghoul stepping into fulfill the function of carrion eaters, but still being far outside the natural cycle, etc.

One other alternative is borrowing heavily from Tolkien and replacing in the Orcs story the word Orcs to Ghouls. So, the Ghouls used to be Elves that were malformed from the continuous torture from a terribly evil wizard (or God or whatever else really evil). The once-Elves have a malformed form of magic within them that makes them undead and hungry for flesh. Also, it gives them the paralyzing touch trait. BUT, even the darkest magics could not fully overcome the inherent fey magic of the Elves and so the paralyzing touch of the Ghouls does not affect Elves.