Ok, Myrtle, I think Filch is likely scummy too, but why him over Snape? If Snape's mafia has figured out some roles, it's possible they did know who the paranoid gun owner is. So while I would agree that if Snape is mafia, it's highly likely Filch and/or Peter also is, but it is not certain. The only certainty, if my theory is correct, is that Snapes is.