I agree that it is Regulus. To me, the big question is who helped him? Does not look as if he could have done it himself, does it?
Kreacher! Kracher helped him, that's why he's so off. Maybe Regulus did a memory charm on him so he doesn't remember, but Kreacher would have done anything for Regulus, he was the "good son". Even drinking evil potion.
Good question. We don't know how long ago Regulus died, but it had to have been pre-Harry. However, the contents of the note make it seem like RAB must have taken the locket post-prophecy.
Dumbledore apparently didn't know RAB had taken it, so I think it's safe to rule out Snape as the accomplice, unless Snape kept it secret from both Dumbledore and Voldemort. Could he have? (The Slytherin tendency toward self-preservation, plus a new appreciation for just how powerful Snape is, suggests "maybe.")
Re Regulus Black. Most Freepers are convinced that
RAB is Regulus. I'm not knocking that suggestion...
but I sure would like the A of RAB explained. I can't
make heads or tails out of that one, so I'm holding
out for there being someone else in Book 7 with those
initials. Just to throw a monkey wrench in here! <>g<>