In response to Beth's theory of Snape displaying loyalty to Dumbledore in a feat that suggests otherwise, I completely agree. Dumbledore was not one to plead, nor beg, at least not in the manner displayed on the night of his death. He was the most intelligent wizard of his time and undoubtedly weighed the pros and cons, thus ultimately deciding that his life in exchange for a more peaceful world was a fair bargain. I have a couple theories that intertwine with Beth's Snape theory but those remain to be posted.Tonight, as a bonus for our "grand opening," I've decided to reveal who I believe R.A.B to be.
R.A.B were the initials left on a piece of parchment stuck between a fake locket believed by Dumbledore and Harry to be another Horcrux. Summarized, the note stated that the author believed he will be dead by the time Voldemort received the note but also that the author stole the real Horcrux and planned to destroy it as soon as possible. We do not know whether or not this Horcrux was destroyed; we can't safely assume that it was, but for the time being, we will.
The author would be none other than Regulus Black, brother of Sirius Black and former Death Eater. After Regulus found out the extent to which Voldemort was going in order to achieve power, Regulus attempted to back out and was eventually killed.Why I believe Regulus to be the mystery author, in addition to the R and the B matching up to the intials (we do now know Black's middle name), is because it is completely logical.
At one point in time, he was a Death Eater, as was much of his family, meaning he knew the a good deal about Voldemort's plans, and possibly more than intended. Whether his discovery of the Horcrux was by suspicion or by blatant explanation, I am not sure, but this theory makes a lot of sense to me. Plus, he was trying to get out, and the best way to attempt to break free from Voldemort is destroy a part of his soul. The author writes that he will most likely be dead by the time Voldemort reads the note. Why? Because the Death Eaters had closed in on Regulus soon after he stole (and hopefully destroyed) the Horcrux. Unfortunately, the reparably-bad wizards finish last...