Hmm, well no-one can say for sure who RAB is...or is it revealed in book 6, I am trying to recall. I remember the huge debates raging over this when OOTP came out. I believe though, that it is Regulus Black, relation of Sirius and Bellatrix, and believed to have been killed in an earlier war with Voldemort. I used to know his full name actually, but almost all Hp readers pretty much concluded that is who it must be.
As to Snape, I am assuming you mean what he did to Dumbledore in book 6? If so, that is one of the hottest debates raging between Hp fans. If you ever glance around and find a site called Mugglenet, you will find a lot of threads there on this, I imagine, and some very interesting. No-one yet knows if Snape is bad or good, though I prefer to think he is a mixture of both and has simply become a victim to his own fatal flaw, whatever that is. Some believe that he made an unbreakable vow with Dumbledore on behalf of Harry, which would explain why he has not harmed him thus far. Some think he did what he did in book 6 because he made that unbreakable vow with Draco Malfoy's mother, and therefore had to step in when Malfoy couldn't do the deed. Some think that Dumbledore was already dying and had asked that Snape do what he do to keep Draco alive. And some believe that Snape still works for Voldemort and did what he did because it was simply time to be rid of Dumbledore. It is really quite a split. I think to really know the truth, we have to wait for book 7, but you have to recall all of Snapes's behaviour in the other books to make your decision on where he stands. There is a lot of evidence...and if you cannot be assed, just find a forum and a thread on whether Snape is good or bad (There are lots!) and read that.
