The TV Tropes page for the 2007 film, Trick 'R Treat repeatedly, (and rather insistently), states that the murderous fake vampire offed by werewolves is Mr. Wilkins, a character earlier introduced as a serial killer. Is there any evidence that they are, indeed, the same character? It has, admittedly, been some years since I watched the film, and I don't have a copy handy to check.
The script provides an answer.
Lori grabs his shirt and rips it open. He finishes taking off the rest himself. Finally, he grabs the edge of his mask and slowly lifts if off, revealing.
Steven Pierce is the name of Principal Steven Wilkins's character in the script, so it seems they are supposed to be the same character (the script also gives his costumed character the name of "The Shadowman").