Why would Sirius need to be killed to keep a secret?

[lone_white_werewolf]

No: HPFGUIDX 163295

Pippin Wrote
> Sirius said something very interesting before he died. He told Harry
> to take the prophecy and run. But Dumbledore said that if he had told
> Harry everything, Harry would have known there was no need to go
> to the ministry. What did Sirius know, and why did he think the
> prophecy needed to be defended? And did someone kill him to keep him
> from revealing the source of his knowledge?

Wouldn't Sirius's loyalty have been enough to prevent him revealing a
secret?

lone_white_werewolf