Okay... so say that Buckbeak's night action went through and Snape was roleblocked... that still doesn't explain why Buckbeak was killed instead of Snape. Sirius was killed even with Buckbeak's protection/roleblock. It's not impossible for there to be a Mafia Hitman or a Mafia Ninja... which means that Buckbeak's protection wouldn't matter anyway. One would think that they would go after the majorly important roles, like Snape's, until Buckbeak started stopping the Mafia's night actions. We have yet to kill a hitman or ninja, so they could have easily just killed Snape and gotten it over with.