It's Always Sunny in Philadelphia: Season 3, Episode 10

3 - 10: Mac is a Serial Killer

Mac has been acting strangely, leading Frank, Dennis and Dee to believe that he's the serial killer who has been terrorizing Philadelphia. So they set out to catch him, but for Dennis and Dee, that means they must first learn to think like serial killers themselves. Frank, meanwhile, is content to let a chainsaw do his thinking for him.