Myrtle:

We also have a chance that there is a Nexus or a Bus Driver, do we not? They could have easily switched who Buckbeak was 'jailing' if Buckbeak really is the Jailkeeper. We can not rule out those possibilities, as well... And of course there is the possibility of a second Mafia. If that is the case, a Mafia Ninja or a Mafia Hitman from the second Mafia would easily have killed Sirius, even though he was 'jailed'.