It's Always Sunny in Philadelphia (2005)

S3E10 Mac Is a Serial Killer

Title: Mac Is a Serial Killer Plot: When Mac shows up at the bar with mysterious scratches on his neck and odd demeanor, Frank becomes convinced that he’s the elusive serial killer preying on young women in Philadelphia. Determined to gather proof against Mac, Frank grabs his trusty chainsaw and enlists a skeptical Charlie for support. Meanwhile, Dennis and Dee engage in some role-playing exercises to gain insight into the potential suspect they’re dealing with, hoping to uncover the truth behind the chilling events.