Theory of dumbledores death

[chladely11]

Hey I'm Chad, just joined this group, and I just finished Half
Blood Prince for the second time.  I was wondering if there's
any debate on whether Dumbledore was killed by Snape in spite
of his treachery or if it was a detailed plan sought out by
the two.  Maybe there was a horcrux whithin Dumbledore and
even Harry himself, and Snape was chosen to kill Voldemort and
Harry once they met. I was also wondering if anyone had any
ideas who R.A.B is and how he or she is connected to the story.
Ty!!