"To the Dark Lord
I know I will be dead long before you read this
but I want you to know that it was I who discovered your secret.
I have stolen the real Horcrux and intend to destroy it as soon as I can.
I face death in the hope that when you meet your match,
you will be mortal once more.
R. A. B."

Is R.A.B. Regulus Black, Sirius's younger brother? Why?
1. His note addresses the Dark Lord a term used by Death Eater's.
Regulus was a Death Eater who turned on Voldemort and was eventually
destroyed by him or his followers.
2. The Death Eaters knew about the Horcruxes as Voldemort said in the
graveyard in GOF to his Death Eaters: "And I asked myself, but how
could they have believed I would not rise again? They, who knew the
steps I took, long ago, to guard myself against mortal death?"
3. Regulus's middle name could be Alphard - named after his uncle.
Other wizard characters are named after same gender family names like
Harry James Potter and Tom Marvolo Riddle.
4. A locket that nobody could open was found at 12 Grimmauld Place and
was discarded. It is thought either Kreacher took it with other items
(Horcruxes) or perhaps Mundungus stole it.
5. Regulus removed the Horcrux with Kreacher who being an elf with
limited magical powers was able to join Regulus in the one-wizard
boat, but two people were required to drink the poison liquid. Regulus
wrote "I" have stolen the real Horcrux, because he didn't respect his
accomplice as an elf enough to be credit him.
6. Voldemort in Book 4 says of his death eaters. "There are 6 missing
death eaters, 3 are dead, one too cowardly to return - he will pay
(Karkorff?), one who has left me - he will be killed (Regulus) one who
remains my faithful servant - he will re-enter my service (Snape?)."
7. The writer of the clue had to be dead given the way the message was
written. Regulus was meant to have died the same year as HP's parents
died, which was 16 years before the message was read. Yet, the message
suggests the writer was aware of the prophecy that Voldemort would
meet his match. Since Harry could command Kreacher, there were no
living relatives of Sirius (such as Regulus) who could inherit #12
Grimmauld Place.
8. Why did Harry inherit #12 Grimmauld Place and Kreacher if they
weren't going to feature in the final book?

Is Snape is working for the light all along? Why?
1.Dumbledore trusted Snape. OK Dumbledore made big mistakes but he
appeared to have had some cast-iron guarantee like an unbreakable vow.
2. Perhaps Dumbledore was dying of the poison and had instructed Snape
beforehand to kill him so that Snape would gain the trust of the Death
Eaters, who would lead him to Voldemort. For this same reason, Snape
took the unbreakable vow to protect Malfoy.
3. Perhaps Dumbledore paralysed Harry under his invisibility cloak to
stop him interfering in the plan and to protect Snape.
4. Dumbledore said "please Severus" - why would the most powerful
wizard plead for his life?
5. Dumbledore was overheard arguing with Snape in the Hogwarts grounds
- was this because Snape didn't want go along with Dumbledore's plan
which involved killing him?
6. In the battles in the Tower, Snape never hurt Harry or his accomplices.
7. Snape owes a debt to Harry after James, Harry's father saved his life.