What really happened on the tower.

[pippin_999]

No: HPFGUIDX 155477

Eggplant:
> In chapter 2 Snape said he helped provide information that caused the
> death of Sirius Black. If that wasn't true why did he say it? How did
> he expect to get away with a lie of that magnitude to the two people
> who know more about the incident than anyone else on planet Earth?
> Kreature went to Narcissa and told her secret stuff about Sirius, and
> Bellatrix was the one who actually killed him. Snape would be a
> absolute fool to say that if it wasn't true. Those two knew the truth
> and if they thought Snape's boast was full of hot air one or both
> would have said something, but they did not.

Pippin:
 
Narcissa's there to beg a favor, literally on bended knee, from someone 
who is "the only one who can help me." She's hardly in a position to call
Snape's bluff. If pressed for details, Snape can claim to have helped 
Kreacher  defect. Since Kreacher is currently in Dumbledore's  custody, 
there's no way for either Narcissa or Bella to check up on the story. 

It's also possible that Snape is telling the truth and that information
on Sirius was intentionally passed with Dumbledore's permission. A
double agent has to give some genuine information to the enemy in
order to establish his usefulness. It would be safe enough for 
Voldemort to learn that Sirius was in  London, for example, provided
Sirius remained in hiding at GP. 

Pippin