Hmm.... I never thought about Harry's scar being a Horcrux. But that would mean that Harry would have to kill himself to destroy it. And now that I think about it I don't think his scar could be a Horcrux. Dumbledore tells Harry that Voldemort was planning to make the last Horcrux at the moment he killed him - Harry - but his plans backfired. (Pun intended.) A Horcrux can only be created at the moment of killing and since Voldemort failed to kill Harry he couldn't have created it!
I think there's little doubt that RAB is Regulus Black. Remember that Sirius told Harry that Regulus tried to leave Death Eaters and was killed because you just can't do that. What none of them realized is that he was actually very successful at leaving!!
Snape has been the most complex and fascinating character in the entire series. He is a man of many contradictions. Noble, but petty and cruel. I think he was acting on Dumbledore's orders when he killed him. Remember the fight Hagrid overheard between Snape and Dumbledore? It may have been the moment Dumbledore revealed his plans to him. And as Harry was chasing Snape at the end with the intention of killing him Snape kept basically teaching Harry! He kept giving him pointers and instructions on occlumency, silent spells, etc. And the idea that Voldemort wanted to kill Harry on his own - when he allowed someone else to kill Dumbledore - isn't entirely convincing. Considering how much damage Harry has caused Voldemort I'm sure he'd want him out of the way any way possible.
As far as why Snape had to kill Dumbledore - because of the Unbreakable Vow and because Malfoy couldn't. A nice twist that this nasty little git Malfoy turns out to have something resembling a soul and simply can not kill! The Vow stated that not only would Snape protect Draco, but also do what he may not be able to. So, to fulfill the vow Snape had to kill Dumbledore. And Dumbledore didn't show any concern over the Vow and Malfoy's plans, of which he was aware. I think he was pulling an Obi-Wan on Harry and I doubt he will return in physical form in the next book. I'm sure his portrait will have something to contribute, however.
I suspect Slughorn might teach Defense Against the Dark Arts next year.
One thing I didn't like in the book is the coupling of Ron with Hermione and Harry with Ginny. I just don't like them as couples... I want to see Hermione with Viktor (who will hopefully return in the next book)!
:thinking: